It's unfortunately very simple in this case, because you purchased the modem such a long time ago. It's been long enough that the seller owes you nothing. Even if you open a case with eBay you're likely to be told immediately that you bought this thing a year and a half ago and your window to settle disputes with the seller has been closed a long time.
Arguably this is stolen property, and the obstacles between you and your refund are large and time-consuming, and probably not worth the cost in time and effort for the amount of money you'll get back. What was it, 50-100 bucks? Are you willing to spend hours of your time filing and then following up on a claim? Is it easy for you to travel to the seller's place of residence? Do you think you can do it without hiring a lawyer?
You could work it another way. You had been using this modem for a year before Comcast decided to inform you that it belonged to them. Comcast really shouldn't have done that.. had they started charging you rent immediately, or better yet, brought the issue up at the time of activation, then you would have known immediately that there was something fishy and could have returned it while you still had a chance. But their dawdling for a year left you holding the bag. They say possession is nine tenths of the law... that isn't really true, but it is possible that by failing to assert their claim of ownership in a timely manner they demonstrated a significant failure to control their property... making the claim of ownership weak when it comes to this specific modem. You might conceivably get a lawyer to compel them to drop the rental charge and allow you to keep using "your" modem from now on.
But again, that will take you days or weeks or even months, possible time spent in your local civil court with comcast, and a lawyer's fee that surely exceeds ten times the cost of the best modem available right now at best buy. Unless you have literally nothing else to do with your time it's probably a poor investment.
