At train shows, most sellers expect offers, some don't and you get to know those guys right away. Most folks don't really want to haul everything they bought back home - it's not cheap to sell at meets, especially multiday meetings like York. So especially towards the end of the meet (anytime Saturday at York), some smokin deals can be had. Sometimes its better to let the seller make a crazy offer instead of you. How do you tell? That's hard - but if it is sellers that you know, you can probably tell when it's time to strike.
And to other's points, sometimes it's an insult to dicker on real low cost items.
A good story I heard - a guy had a caboose on his table, decent shape, for $8. Guy came up and offered him $2. Seller took the caboose, placed it on the floor, and smashed it to bits with a hard stomp. Strong message to buyer.
Jim