Start with Ebay , do a search of your model, and even if it comes up with zero, check as ped suggested
There's ' completed listings ' and ' sold listings ' down the left hand side. You want ' sold listings ' , because the other option doesnt mean they sold, just that the auction ended without it being cancelled by the seller
If Ebay comes up with quite a few sold listings of your bass, you need to be careful about the sale price. Make sure those that sold are in the same condition. For basses that sold too cheap, was the seller desperate for money ? Its hard to tell.
Its just a guide, but if no sales come up, you'll just have to make your own price up. Go high, because you can always haggle down. You cannot do it in reverse