Legally, the router is not yours to sell. It remains property of Virgin Media at all times. Of course, that doesn't stop people from selling them anyway, but as Chuck has pointed out, you won't be lining your pockets with gold after the sale, assuming you sell it.
If it's a Netgear router, which many of Virgins are, it will be crippled by their proprietary firmware anyway which any prospective enthusiast buyer will want to flash off it, which can result in it being totally useless, so they won't want to pay very much for it.