I suppose in the first instance I would make sure it was actually a good one. So apart from drooling over it and getting too excited, I would closely examine every bit of it and then play it, both unplugged and through an amp.
If all was to my satisfaction and it really was the one that I had always wanted, then I would be happy to sell/exchange other instruments to fund the purchase.
Finally, it's highly unlikely that you will never want another bass, although that is a good line to use when discussing these sort of purchases with partners/wifes/girlfriends etc.
Good luck and enjoy the decision making process.