If you're selling secondhand stuff, the best tactic is to do a bit of research and see what similar items are going for (ebay, craigslist, collector boards, etc). And of course make some determination whether you're trying to make money or just get rid of it.
Real estate is a massively complex notion, but there are a few basics. It's worth the cost of the land plus improvements (such as buildings). Since structures are generally built to last there's some depreciation, but it maintains fairly well assuming proper care, although the type of building can make a big difference, too.
There are lots of tools for finding out costs. Zillow.com will give you a pretty notion. You can also look at the county auditor records which will tell you the assessed value (be aware that price is almost always low).