I find that when I see something that I want for my collection at a train show I should attempt to buy it then. I have seldom seen the same item that I want at train show after train show. I either buy it then or it is gone forever.
I always try to negotiate by asking what is the seller's best price. Many sellers will automatically take $5 to $10 off a low price item. I can usually get $25 to $50 off on a high price item like an engine. If an item is $5 or less I pay the asking price.
It never hurts to ask a seller about a discount on his or her asking price. You see it done on the "American Pickers" TV show all the time.
On the flip side, I always expect buyers to ask for a lower price when I am selling. I am usually willing to give a buyer a lower price. I don't make a living selling trains and I usually sell items at a lower price than I paid for them. That is today's market.
NH Joe