My understanding of the eBay rules is that a bottle containing alcohol can only be sold if it can be argued that the bottle is worth more than the contents - i.e. that it is a 'collectable container'.
As a result some entries for port have included wording like "the bottle is sedimented, and therefore undrinkable"
Whether the vendor believes this or not is debateable
There is a provision in the rules for wine sellers to register with eBay - presumably they can then sell anything - but I've never seen evidence of one trading.
In practice, eBay do not vet goods offered for sale as they are entered, so bottles do appear. But every few days they swoop and cancel any entries they find.
When sales have completed, prices have been absurdly high, for wine of very suspect provenance - so a place to sell, not to buy!
On a point of UK law, members of the public can only sell alcohol to licensed premises unless they themselves are licensed. One can sidestep this (to a degree

) by making out a receipt to the buyer's local pub, so officially you are selling the wine to the pub, who then sells on to the buyer, even though in practice the pub never handles the bottle or the cash.
One should not be too blatent about this though... 8)
Tom