Read the subject heading. These should be the minimum guidelines for anybody selling anything on here. Look through their posts as well. Do they actually voice anything thought-provoking or contribute anything to the community? Or are they all marketplace posts? Notice how virtually any graphics card or other electronics product/gadget sold on here is by some dude with under 50 posts, and if you check, most of their posts are in the marketplace section. Or, make the seller ship first, and only send BTC when you receive your product. This really isn't that hard.
If they have positive rep, check to see who gave them the positive rep. Was the account linked to the pos. rep post created in the same time frame? Are their post counts similar? Are the majority of their posts also in the marketplace section?
Does the seller refuse escrow? Does the seller come up with weird bullshit reasons for why he can't provide real proof of something? ThyDude came up with some bullshit "I'm in legal trouble and I don't want my handwriting posted anywhere online cause it could get me in more trouble." HELLO?!?!?!?!?!!?! Apparently, Dalambchops had a positive transaction with this guy. A good scammer will do some legit transactions, build rep, and hit the big scams (notice how ThyDude sold like everything in his goddamn house?).
Here's a novel idea. Ask for a phone number. Call them. Talk with them. Bitcoin isn't completely anonymous anyway, and it's far less anonymous on a forum where you are communicating with each other on a daily basis. Honest buyer? Honest seller? Then what's the big deal with anonymity? In my opinion, if you want to remain anonymous, I don't trust you anyway.
I've had 2 buyers buy from me. I could have just as easily scammed them too (obviously, I didn't). If they would have said, "Hey, do you think I could call you and we could talk about this?" I'd be like, "Why yes! I understand your concern, and I am concerned for the same reasons." This doesn't mean you need to show your driver's license or anything ridiculous. But honestly, if you get scammed on here, you did it to yourself. No one forced you to send your BTC. You chose. Besides, what would even a cruel-intentioned person do with your phone number? Text you? Text you twice? Text you thrice?