Wow do we get awfully fil-also-fickle about these discussions.
A couple ideas that both revolve around the idea that the less scrupulous you are the more you tend towards easy marks.
If we as users simply agree that all selling posts NOT in the appropriate sub-forum are to be reported, that alone will do more than just about anything else. When this happens, the thread can be locked, a canned message stamped to repost in the proper section, and left at that. Only serious sellers will go through the trouble of reposting; frauds will fear they've been caught and get out. I feel that self-policing as a community is the most practical way to handle things.
Another protection to add to that which takes a small effort on
@Bonsai Nut's part and is yet to be mentioned is to keep the buy/sell section off the front page feed. You can only see it if you're looking for it, which cuts down on impulse buys, and a fraudulent seller's potential audience.
I dislike this idea myself because even if I'm not buying seeing the threads of sellers as they pop up gives me the ability to casually keep an eye on the market, which benefits all of us when it comes to fraudulent sellers. Segregating the sellers also segregates the rest of us from the knowledge that helps us identify nerdowells.
Just thought I'd bring this concept up.
Overall, my vote is for a 30 day wait to sell on the forum, and all selling posts must be in the proper section. Along with the current standard of 10 post minimum to DM (because you're not likely to buy or sell without some DMs) I think creates allot of protection actually. A note in bold bright letters can be added to the new thread composition screen. It'd be annoying to look at for some regulars, but gives no excuse for not knowing the standard, then the rest is on us. Only needs to be a couple lines in bullet points.