You say this like it's a problem.I got a bit out of control this year and managed to go from 4 instruments to 34.
Step one - get the hell off of Talkbass. You probably never even knew you wanted all that crap before you came here. Step two - sell them the way you bought them - one at a time.
Unless you need the money, my criteria would be 1) the bass you would lose the least money on (or the bass you would make the most money on), 2) the bass you could replace the easiest 3) the bass you play the least. I have a '66 Tele bass that I never play. I would make money on it if I sold it, but I can't bring myself to get rid of it because I'll probably never have a chance to own a '66 Tele again.
Good luck. I'm going to try to get under 10 myself.