As a collector of many years I have seen most of the things done to get a collector to part with his or her hard earned money. Due to the popularity of these models and with Matchbox collectors being variant collectors, the hobby was and still is to some extent rife with fakers and frauds.
Probably the easiest one is the wheel swap. Obtain a genuine model and swap over the plastic wheels from another to make a scarcer variant. Couple of minutes work and make a few bucks extra as these do not sell for as much as they used to due to the previous fact no one trusts them nowadays. Very easy to do and one for the amateurs, but if these interest you check the area on the wheel around the axle hole - any crazing to the plating or damage - leave well alone or return for a refund.
Fake labels. In this day of great printers and decal paper it is all to easy to either photocopy a decal from one model, reproduce it and stick on an entirely differnt model. Only answer to this one is know what type of label was originally applied and is the quality the same - if not leave well alone.
The 'Friday' model. So called because they were made by employees at the Matchbox factory to raise a bit of extra revenue. Get a standard model off the line and apply the labels from another model to it and call it a 'trial' model but basically they are stolen models. As an example I have seen so many different coloured models with 'Captain Morgan' or 'Nestle' labels on it described as 'trials' it has become a joke. Some of these models were issued before these logos were even issued it is that bad. Some collectors like them but if you do, do not pay to much higher a price than that asked for a standard model.
The REAL FAKE. These are the worst as they usually involved high priced items that are rare. I have seen all the tricks used and even been caught myself recently so they are hard to detect until you get them in your hand.
The rivets can be drilled out and then tapped and have new ones cut from another model reglued into them so adding a rare base to a normal body or vice versa. Repainted and reassembled models or those with plastic parts recast from new moulds taken from the originals but they are never the same usually.
The fakers of these models rely on the fact that no collector is going to try and pull an expensive model apart or that the model is that rare that most collectors have never seen one so cannot compare to something else and sit there untill the day comes and they want to sell and the buyer has a wee bit more knowledge or someone the same comes round to view your collection and tells you the bad news to late by then.
If you complain there is always the standard get out - "They were genuine ones I sent so you have swapped the bits so I cannot refund" - yeh sure - rare bits abound everywhere to swap them with. Or the "I didn't know as I didn't check it that well, you say it came apart as it was glued" this is accompanied by the usual astonished gasp. Then the usual "send it back to me and I will check it" yes great - now you have no model, no money or any proof. Never, ever send it back, instead take very good photographs of the item, preferably in Macro and keep them as this will be proof for the 'swapped' statements by comparing to the original sale ones. Contact Paypal if used or Ebay and if no satisfaction then your solicitor because basically it is theft through fraud, whether they claim to know what they have sold or not - it was advertised as rare and genuine. Keep all original Ebay pics as proof as these can be blown up and show tiny details not usually seen by most that provide a perfect ID of the item when compared to your ones.
This is not to be confused where a rare looking one is slipped into a job lot. Usually a low start bid and tiny pics but collectors being what they are will outbid one another to get this 'rare model' in the bunch which usually turns out to be more garbage and the seller gets a handsome profit on his lot he paid very little for. Don't get me wrong on this one as there are genuine sellers who know nothing about these models and just list them all together and you can get lucky but it happens very infrequently.
My advice is to get anything like this checked with a reputable dealer or person who has the knowledge even if it means posting on somewhere else to get this done. For really expensive models there is a service in the US that will check the paint ( they used to take a microchip off but even this is no longer required now) and will match it to known Lesney / Matchbox paint samples and formulas in use at the time of issue (these have been gathered from many common and rare models to build up a database) and they can even tell you how long that paint has been on a model.
The power of this last test is amazing. I have a mate here that I chat to at swaps and for years he carried around 2 small buses, both green in colour. Everybody said they were repaints as all Matchbox buses of this period were red. He finally sold them at a good price, for him, to 2 other collectors who trusted him and they later had them tested when this process came out and gleaned some facts about them from other sources. They proved 100% genuine and were apparently made as a trial for NSW as all buses were green here then and they couldn't sell many red ones . One of these models sold recently for around $7000!!!! so it works in your favour sometimes.
Be careful of what you buy and know your hobby. There are many good sellers out there and they are quick and easy to learn who they are and most generally get a higher price than normal as the items are as described and the service is excellent. Stay away from the sellers from hell.