yeah you can do that, but I would offer a couple bits of potential concern.
First off, if you're buying a phone off ebay, either do it through an ebay company (not just a random user) or if you do buy from a random user, make sure that it's not a phone they got from their 'contract renewal.' If you buy a phone that was purchased with a contract renewal (as in an upgrade) you'll need the account information from the person who had the phone, which if you do off of ebay, i'm sure they will be less than likely to give you that information.
Secondly if you are going to buy it from an online retailer, be it an ebayer or not, it'll go for around $500, whereas if you wait for the contract up it's around $150, but in a year the phone will be replaced by something else.
As an alternative, go to verizon and check the status of your phone upgrades. they changed it and you're eligible for an upgrade every 1.5 years, not every 2, and you can transfer upgrades to different numbers on the same account. So for instance if you're on the same plan as your son (which I don't know why you wouldn't) if you're eligible for an upgrade and don't plan on using it right away you can transfer that to his number and get the droid 2 for the $150 price, which would end up being easier and cheaper than buying it off of ebay and dealing with potentially disastrous problems.
