Your instincts. You start talking to a person, and by reading what he writes, you can learn his style, and you can also find out whether he's just trying to act normal to get you into something you do not want to or if he/she is someone who is trying to make a successful trades here on sythe. The best way to tell if a leecher is legit is by talking to them. Imo I would never trade someone who has compeltely 0 grammar or who uses these coolboy slang talk, because I don't feel any trust towards those kind of people.
The other thing I'd like to mention is:
I agree with Unb4nned. When I'm looking for trades I look for people who I've heard of, or whose thread has massive amounts of posts e.g. in a pin shop, knowing he/she has already done many trades.
And the 3. thing is:
Post count? Join date?
So if I joined in 2007, never logged in for 2 years, then 2 months ago, I started wishing all the account sellers good luck and giving free bumps, will that make me more trustable then I am now? I don't think so. If you consider trust as post count, you are an idiot tbh. And also if you do such, you would have to look through the costumer's posts, whether he makes smart, thoughtful, meaningful posts, or is just trying to raise his / her postcount.