I have a few tips and anecdotes I can share...I usually attend shows as a seller, but set up next to a buddy so we can shop as well, with one watching the others table(s).
I spent 31 years selling TO car dealers so, yes, I learned a few things about negotiating. Dealers may think they are in the train business. We're not. We're in the people business, and need to treat buyers the way we would like to be treated on that side of the table.
First, I always price things with 10% wiggle room, so my prices are fair to start with. When somebody picks up a $10 item and asks if I can do any better, I make a point of saying, Thanks, $20 would do better for me, I appreciate it. Then I laugh, and ask them what they have in mind. When I get lowball offers I refuse them and say thanks. I then throw the item in a tote so when that person returns near the end of the show looking for it I smile and tell them, Oh, that sold...I got full price, too. Maybe next time they'll think twice.
Somebody asked me once if I did volume discounts, and I said Sure. They chose four $1.00 junk box items and asked me if I'd take $2.00. I said, No thanks, but thanks for stopping by, effectively telling them not to bother returning. Sometimes personal satisfaction is as good as a sale. I always have boxes of $1.00 and $2.00 items I'll say, No I can't do better on that, but I can throw in 2, 3, 4 (junk box) items, whatever. Not only do I hold the line on price, but I also go home with a few less items, which is always the goal.
Regarding eBay, there are fees and commissions to eBay and Paypal. Regardless of where you sell, there are costs involved. Also, sellers love to mention eBay prices, and I always ask if they are looking at current listings, completed listings, or sold completed listings. Anybody can ask whatever they want, but the best price guide is seeing what things actually SOLD for.
Last thought for now. If I'm a vendor and I see something I like on another table, I ask the seller to put it aside, ask them if they swap, and invite them to come see my stock. We end up trading 9 of 10 times.
The goals are to have some fun, make a few bucks, and go home with less or different stuff. Happy Holidays!