Take cash, and remember ALL PRICES ARE NEGOTIABLE!
Look around if you see a better price on some other table try to talk the guy down. Don't be afraid to walk away, you can come back later. (Sometimes)
Look it over good (make sure it is unloaded) and look down the barrel.
Take note of any nicks or scratches.
Most Important HAVE FUN!!
Ok, so I went to both the show I mentioned in the post and the Harrisburg show. The first show only had one PSL that was in crappy condition and the guy wanted $950 for it. He was also feeding me BS about its accuracy. No chance I was going to buy from him. The show today had one in excellent condition for $850. I was going to buy it, but the guy told me that he didn't want to sell it because he isn't familiar with MD gun laws. It's a shame that MD has such a reputation for being gun-Nazis. I told him that the only restriction was no mags over 20 rounds, but I understand his point of view. To make the best of it, I took the opportunity to join the NRA.