We wish this was an easy one to answer, unfortunately it’s impossible to give a good price for every product on the second hand market. All we can say is research your product.
If you think an offer is too good to be true, it probably is!
There will always be great deals run by shops and retailers, many of these are for valid reasons but there will always be products that are less effective than others.
Be informed, be aware, research, talk to friends, shops and on the Boards forum, and suss out what you really want before parting with your money!