Just be careful buying a used cable modem. I'm sure you know mostly everyone here, but buying a used modem from someone off eBay or another source may be a bad idea.
If the other person didn't pay their last bill with say, Comcast, which services a good portion of the country, if you're in a Comcast area and you try to activate that modem on your account, it'll be blacklisted.
Also, if the other person uncapped their modem, they could've also blacklisted the modem on service providers that way too. Also, modems can be fried by uploading different firmwares...
It might be worth it to get the new SB5120 (great modem), brand new for the $40, and not go through the aggravation.