Selling used Equipment/Appliances
There is a company which offers a service on ebay for about $3.50 a month (first month free) which allows you to get information about all completed sales on ebay for a particular item. So you would enter in what you had, and see what prices they sold for, and what percentage sold and other interesting data. If you are interested, let me know and I will see if I can find the website for you.
You can also get this sort of info manually, by searching for your item specifying 'completed items only'.
This is probably your only real option other than 'guessing', as if a stove is worth 120 in a local store, it may only be worth 30 on ebay because it might cost 90 to ship it