well if you're very keen on keeping the same number you can always sign up for grandcentral (
http://grandcentral.com/) and get a "universal" number that rings any or all your numbers. You can't make that your sprint number, but you can give it to people temporarily until the month is up.
Btw, I did that ebay thing for my 6700 and two other phones which i got for free and I got a month of service for free too (ask about that). But,, you can't do sero on those lines, you can't apply any employee discounts, you can't change your plan until a month is up and I also remember that I could not do an ESN swap or even a transfer of liability for 6 months so make sure to double check all those conditions so you get all the credits properly applied to your account. The ebay seller i got it from is no longer registered, so I don't know if they all follow the same rules. Also make sure that there's some kind of number or person you can reach by phone before completing a transaction.