Can I purchase a cablecard from ebay and ask Verizon to activate it without paying the monthly rental fee?
No, unfortunately they won't activate STB's and Cablecards like that. That card is likely either stolen or not returned as it should have been to verizon, which means verizon would flag it as such in its inventory.
that thread gives good info about it, it talks about stb's, but for all intents and purposes they should be saying STB's/Cable Cards
Thanks for the information!