Negotiating is an art form and can be difficult. A large part of it boils down to how bad you want it and how bad they want to sell it.
Many train sellers at York are very local and have less motivation to sell their goods. They usually have the same stuff every show and may be there as much to buy new stock to sell at smaller train shows or to socialize.
Some sellers are long distance and really want to unload. Some buy collections and make their money on selling the 1st half of the collection on line or locally. They are at York to liquidate as much of the balance of inventory as possible. As you attend more York's you will recognize certain sellers and just walk on by.
On your first York you can just quickly look at a few prices on the table, and make a decision to stay and shop or move on if the prices are normal or high.
Remember time is fleeting and the entire time you are shopping, other people are buying things and taking those purchases to the car. So you will never see those items on that table.
If you find something you want, try to find other items to bundle with it and make an offer. You can be aggressive with the offer, but too low can turn off any future discussion. The longer you look at the item and fondle it, the more you are telling the seller how bad you want it. Getting rid of bulk for dealers can be as important as recovering the cash to reinvest.
Always be polite and cordial. You may eventually come back and buy something at full price cause no one else has one.