I was talking subscription based electronics (anything that requires a subscription to fully function) in General but Best Buy, Target, and Walmart all sell cable modems.
Buying subscription based electronics used/second hand is always a gamble as they can be blacklisted by their respective service providers for something as simple as a delinquent account. I had to buy a new modem because our cable was in someone else's name, person moved out and did not pay the bill so the modem linked to that account (which I bought in the first place) could not be activated when I established service in my name because the modem was associated with a delinquent account. I had to buy a new modem to the tune of $150 at the time (2006). I am glad this person is no longer living with me and my mother.