1) Don't use a published price guide. I've found they are wrong. They don't keep up with the supply/demand curve. Know what an item is worth from input from many different sources.
2) Know what you are willing to pay for an item based on #1. If it is below it, buy it on the spot. If it is above it, wait....chances are you'll see it again.
3) Don't pass-up an item you are willing to pay what they want for in hope that you will find it elsewhere. If it was a great deal to you, it will be a great deal to someone else right behind you and by the time you get back, it will be gone. I cannot remember how many times I violated this rule and lost out.
4) If you think you are getting a great deal, don't try and get an even better one.
5) Don't stand in front of an item debating if you are going to purchase it - someone might just reach over your shoulder, pull it out, and buy it literally right from under you. Have seen it happen.
6) See #1. Wild fluctuations in price on "identical" items, within the same isle, same hall, and different halls. Of course, the sellers will tell you that theirs is different. Theirs is unopened. Only gloves have been worn when handling. No pets, no children, no smoke, no alcohol, no drugs.... Theirs came from the estate of so-and-so. Theirs has been on a shelf for 30 years and hasn't had power applied. Theirs hasn't seen a track - ever....etc.
Examples:
1) Was missing the LTI 6464 Series 1 remake set of boxcars. I "borrowed" a Greenburg guide at a local table. If I recall, it said they were "worth" $650....maybe in 1995! It was like 2012. What did the guy want? Around $650! Went 2 isles over in Red/White hall and found the same item, this guy wanted $275. Passsed again. Went home and bought a set on Trainz for like $120 and still probably overpaid. Bottom line, know what it's going for on-line.
2) Conversely, saw a MTH UP RPO car I really wanted. It was on my list. It was $30-35. It was the first one I saw in the Purple Hall. Wanted to hit a couple other halls first. By the time I got back, it was gone. I knew I was willing to pay $60. Couldn't find another. And after seeing one for $30, wasn't willing to even go up to $50. Still bugs me today for not getting it.
3) MTH PS2 major road name 6-axle diesel (well used) was not labeled. Asked for price. Was told $150. Knew it was probably worth around $200+ to me, as it was a road name/number that I was missing and was on my "look-for" list. Saw one or two on e-bay for $250. For some stupid reason, I tried to get him to go down in price just a little. He looked at the item, and then it struck him that the price he gave me assumed non-powered and since it was powered, he raised it to $250. Turn-around is fair play. Needless to say, he wouldn't sell it for $150 and I wouldn't buy it for $250, and it was clear we were not going to meet in the middle. Had I kept my mouth shut, I could have had it for $150.