It seems that the more successful a site gets, the more scammers target that site and try to catch out unfortunate buyers and they are getting more convincing all the time.
It used to be that you could spot a scam advert a mile away, the language used, how the descriptions didn't match the photos and of course the price always gave them away. These days though, scammers copy photos and descriptions from genuine adverts and use those instead, so the adverts can look 100% genuine.
The first indication is when you contact them via email and the reply you get mentions that the car is located somewhere else, they try and tell you that it's not possible for you to see the car because of some circumstance - always insist on seeing the car before you part with any money. If the car is in another country, tell the buyer that you would like a friend to come and see it for you, or set up a skype call and get them to show you around the car via skype. If there is any reason why they can't let you see the car then take this as a good indication that it's a scam.
Often the scammers will offer to list the car on eBay with an agreed 'buy it now' price and you can buy it through eBay and pay the deposit via payPal - trying to convince you that this is a safe way to buy because you're covered under the eBay terms and conditions, this is another way they will try and scam you out of either just the deposit, or maybe the full amount.
Do some research on the seller, copy some of the text of the advert and Google it - if an old eBay advert comes up for half the price then it's more than likely a scam advert. If the same advert comes up on other sites, take a look at the username and see how long they've been members of those other sites, if they're all new then tread with caution. Also look at their email address, is it a generic email that shows no identity? Does the name actually match who they are calling themselves?
Ask specific questions about the car/part, something that only a genuine seller would know the answer to, or even ask a trick question (why does it have X if it is a 1967 car?). A genuine seller will probably be able to answer correctly or give you a genuine answer whereas a scam seller will probably avoid the question and try to pressure you into making a payment to secure the car.
If any seller wants you to go to a website and fill in your details, do not do this, these sites are fake and steal your information. If any seller offers to ship the vehicle to you for free, again this is probably fake.
Of course if you spot a scam advert, try to let the site owner know so that it can be removed and always remember the golden rule:
If it looks too good to be true, it probably is..!