I consider the used price of an item, its age and condition. Also the supply and demand. Forty to sixty percent would be a high offer and twenty to thirty percent a low price.
I do a lot of bundling items when at a second hand sale. Just recently, wife and I bundled a list of items we were interested in and made an offer to the owner. He accepted. Included in our purchase was a nice electric bicycle which cost me zero.
I purchased a nice electric wheel chair and repaired it for a total investment of $150.00 and a friend wanted to trade me a like new 10,000 lb. Blue Ox Adventa II tow bar for it. We made an even trade and both were/are happy and both of us have enjoyed many years of service from our items. He had no use for the tow bar and I had no use for the wheel chair.
A bargain is when both the seller and the buyer are happy with the transaction.