This is a general buyers guide, describing not only counterfeits, but things to look out for in general. There's been numerous guides out there to fakes and the best way I think is to look around for some pictures of fakes and compare them side-by-side. The tell-tale sign is the lettering on bottom but I've noticed that on a lot of fake ligne 2's, the ridging on the top of the flint cover is almost 'u' shaped in the middle, kind of like a ligne 1's, but to a lesser extent. A real L2 is always a downward slant and never slants back up again. Older ones had a shorter slant. I've been able to use this method of determining fakes without having to actually see the bottom of a lighter, which is useful if you're buying online

St Dupont Lighter Serial Number Check
St Dupont Lighter Serial Number Check Free
How To Spot Fake St Dupont Lighters
St Dupont Lighter Serial Number Check Online

The serial number appears on the bottom of the lighter, so if an ad selling the lighter doesn't show the bottom, the lighter in question is probably a fake. Is my S.T.Dupont lighter. Stihl Ts 400 Serial Number Location more. You should check some of. The reason I showed the pic from the bottom is because this person claimed that the serial. St Dupont Lighter Refill Gas. The serial number, The first gas lighter made by ST Dupont was introduced in 1953 under D57 code mark. ST Dupont Collectable Lighters, Pens and Humidors including model number, serial number, some item description. If sending us pictures please send.

Visual Cues

fake

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Another thing to remember is that real vintage ligne 2's are somewhat easy to spot if they have the shorter style filnt ridges. I've not seen fakes like this. Vintage Gatsby's also share this trait:
This isn't an entirely reliable way to tell unless the picture shows that portion of the lighter clearly. On some real ligne 2's it looks like it's slanting back up if the photo is at a slight angle, but they should taper downward and stay relatively the same size until the end. They will not get any higher and if you notice the ridges getting higher towards the hinge side it's probably fake.
Serial

I'd expect most older style ligne 2's to have the older emblem, which has dupont spelled across the bottom. The newer lighters have either just a D with wings or no emblem at all. I have not seen any fakes trying to emulate the older style emblem so generally they are trying to copy the one with just a D and the wings. Generally the ones with no emblem come in the newer black/purple boxes although that may not always be the case. Limited editions often don't have emblems either, and will have an issue number rather than a serial number, such as 0001/1000.
Generally the fakes with lacquer look somewhat plastic-like in the pictures, almost looking too shiny. Also if it says lacque de chine and it's not lacquer, it's a fake. Anything with FK in the serial number is fake, as well as lighters with the serial number 06D08A8 or similar (a couple letters/numbers swapped but otherwise the same). Serial numbers are generally 6 or 7 digits long but never longer and should end with a number.
Pricing Estimates
All estimates are USD and are for reference purpose only. These should not be used as actual pricing guidelines, but rather as minimum prices to use when avoiding fakes.
Ligne 1
Ligne 1's are often sold between $50 and $250 used for most common models, depending on condition. New old stock can often be found for $200-400 range. Damaged/heavily used ones can sell for extremely cheap sometimes - $30 - 50. Note that there's a few generations of Ligne 1's, the oldest have a slot on the fuel cap and a older style flame adjuster valve:

old flame adjuster

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old bottom w/slotted fuel valve

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These are pretty common and generally aren't worth as much due to their age and older mechanism. These lighters cannot use the double flame and/or pipe fuel nozzles that the later Ligne 1's and ligne 2's can accept. These are probably the cheapest of all used dupont lighters, and you can get one of these for less than $100 easily, regardless of the pattern and size. Some of the ones with heavier gold plating might be worth more, but the later heavy-gold-plated models are still more desirable yet.
Most of the newer style will use the pull-down style fuel cap and the newer style flame valve:

new style flame adjuster

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St Dupont Lighter Serial Number Check

flip down bottom

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However, even some newer Ligne 1's have the older style slot, with the newer style flame height adjuster. This is generally only seen with certain plating/finish options, typically diamond-head patterns in palladium. However, the tell-tale sign as to whether it's a newer or older Ligne 1 is always the flame height adjuster valve. The newer lighters with the slot generally have newer markings, and tend to use one of the heavier platings, like this one:

new lighter with heavy plating and old-style slot.

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Ligne 2
Used Ligne 2's and Gatsby's can range anywhere between $150-350 used for most common models. I have seen a few in bad condition sell for as little as $80, generally worn silver plated models in undesirable/outdated patterns, or badly cracked lacquer, but this is somewhat uncommon. You can find often find new-old-stock or like-new in the $350-500 range. If you see a like-new Ligne 2 for less than $300, something is probably not right. Also note there's a lot of fakes posted in the $150-300 range. In terms of age, there's not as much that varies between the ligne 2 and gatsby models over the years, generally condition and desirability of the finish is going to be more important than age alone. As mentioned before when identify fakes, the older models tend to have shorter ridging on the flint area, but I have not seen this to effect the value, especially if the condition is good. Mechanically and functionally they are all pretty much identical to my best knowledge.
Limited Editions
Limited editions are generally not cheap, regardless of whether it's ligne 1 or ligne 2. While in general ligne 1's are less expensive, the design can easily make up for it. It's rare that you will see a limited edition under $500, and usually they will sell for $800 at a minimum. Commonly counterfeited limited editions include the james bond 007 lighter with the bullet spark wheel, and many variants of engraved + lacquered lighters, generally resembling art deco, medici, nuevo mundo and the like. There's also a lot of fakes with dragons and other such designs. Some counterfeits were never real designs at all, but resemble a limited edition design. Most if not all real limited editions can be found at http://www.dupontcollector.com for reference.
I've also heard of custom-order dupont lighters, typically in smaller batches between 100-300, often for a specific purpose, generally a company, brand or event of some type. These are not very common and I'd imagine the person selling it would more often than not be the original owner, or know some history about it to prove it's authenticity.
Almost all limited editions will have an issue number, rather than a serial number, like 0050/1000. In addition, the markings on the bottom are often slightly different, and there may be no dupont emblem at all on some limited editions, although almost all still have the 'ST Dupont made in paris' text.
Ligne 1 Fakes? Not usually... Dupont

I've never personally seen a ligne 1 fake, so it's usually safe to buy used ligne 1's, they also generally sell for pretty cheap. However, I have seen counterfeiters store listing ligne 1's but the pictures appeared to be stock pictures of actual ligne 1's, listed around $30-50. My gu