It is the shipping costs that may keep you from listing on ebay.
But to find out your value, look for your pc in used condition.
Filter on completed auctions.
In green you will see what a similar piece actually sold for.
If it's a normal size tower rather than one of those small form-factor towers, I'd imagine the computer's worth maybe $150 to $200 assuming that a full, legitimate Windows license is included, based on a system I sold fairly recently.