Charge monthly. That's better for a hosted service anyway, as it matches up better with your expenses. If it stops working, stop charging them. Refund months th

Charge monthly. That's better for a hosted service anyway, as it matches up better with your expenses. If it stops working, stop charging them. Refund months they've already paid for that you won't be able to fulfill. Make sure they know up front that this service depends on EBay not being a jerk. That way you'll still get paid for however long you DO operate it, and not have to feel guilty about keeping the money when it stops working.