Man that sucks. There's always some asshole that has to come along and ruins shit for everyone. I've done a shitload of high-dollar forum trades and sales without incident. There were a couple of exaggerations here and there, but overall pretty well done.
The most common thing I found was amps coming with used up old tubes. For some reason, people don't like to be honest about the tubes and will fight you tooth and nail over it.
Even so, I'm increasingly and rapidly becoming suspicious as I've seen a dramatic increase in sour deals. Furthermore, there's always some asshole who expects you to give your shit away dirt cheap and pay them premiums for their shit. I can't count the number of douchebags who feel so entitled that they get mad and email/PM you with shitty mesages. Some even go as far as to bring shit up months later in unrelated threads because they're still holding grudges about it.
One thing that I find quite alarming are the counterfeits going around. Some dude emailed me some pics of his fake-ass Les Paul where he fixed up a Gibson neck and then put his own body and everything together. He did some good fucking work, but man that dude is a fucking crook. You know he'll pass that shit off as a VOS and make a ton of money on a piece of shit Gibson neck he bought off of Ebay.
What stuns me about it is if people like him have that kind of skill, then whey the hell go around shafting people? Unfortunately, the answer is that no one really gives a shit any more.
I say just watch your ass and be especially aware of counterfeit shit. There are certain advantages to buying stuff from authorized dealers, even if it costs a lot of money.
Here are a few things I look for. It's not a comprehensive list of guidelines by any stretch of hte imagination, and some people might not agree with my personal rules. They're my PERSONAL guidelines so take it for what it's worth. So far though, I've done about $28,000 in trades in the last year or so without incident.
1. If something is too good to be true, it probably is. Don't be a cheapskate and think that you're going to take some ususpecting newb for a ride because you will find a rude awakening.
2. If a dude doesn't post details about stuff, don't trust him. He has too much too hide. Sometimes getting details about gear for sale is like squeezing water out of a fucking rock. It's like you have to ask all the questions and nothing is freely offered. Don't trust people who aren't forthcoming with information.
Sometimes people offer too much information. That too can be used to mislead you or distract you from undesireable facts. Make sure you prioritize what's important to you.
3. Beware if they're not the original owner. I can't tell you how many times an amp might get passed around and traded on a forum only to sell for twice as much as it originally posted for. There's always some dude who buys something on the forum and then turns around and posts it for a lot more than he paid, using the old "it's just not for me" routine. They know very well what their intentions were to begin with so don't make them any richer for it.
4. Stop perpetuating the "vintage" market. I'm sure I'll catch a bunch of shit from old Fender guys and whatnot, but to me old shit is plain old shit. I really don't give a damned what people think it's worth. If it's used and old, it's just used and old. I don't get these guys who pay out the ass for old-ass Soldanos, Fenders, etc... I'd rather buy something new for the same price or slightly more that comes with a warranty.
Man I can't tell you how many times interpretation gets mixed up when trading/buying "vintage" gear. If you're going to buy old shit, just be ready to fork out a bunch of dough for parts, NOS tubes, etc... if something goes wrong.
Oh yeah, and stay away from modified vintage shit. If you have a badass old Fender, Marshall, etc... and it's been modded, don't expect to get 1/2 of what it's worth without the mods - even if the mods are an improvement.
As far as I'm concerned, 80% of the time the shit being sold/traded on the used market is not worth anywhere near what it gets valued at. Every now and then you run into a few gems, like those rare guys who have an old Les Paul in their basement or something like that, but those are few and far between.
5. Be your own judge of value. Don't let someone hussle you into giving them what they think their shit is worth. When it comes down to it, they have to sell the shit at the price they expect, pay all the fees and shipping associated with the sale, and they almost never get nearly what they expect in the end. The reality is that you have to find a buyer, they have to buy at your price, you have to pay fees associated with the sale, shipping charges, and you have to support the buyer if something goes wrong.
In the end, those guys almost NEVER get what they expect. 1/2 the time they know it but can be very forcefull and convincing. You just have to be your own judge of value.
Like I said, these are some of my PERSONAL guidelines and experiences. They might not be agreeable to you but they've worked well for me and I'm pretty sure I do more trading than the average person.