Technically, ebay does not allow the sale of 'Electronic cigarettes (e-cigarettes) and components'. (Ironically, nicotine patches and gum *are* allowed.)
Ebay does not really patrol its site looking for e-cig stuff. But they will remove the listing if someone reports it.
PayPal (which is owned by ebay) has the same policy. So if you buy something from a seller and have a problem (don't receive item, DOA, etc.), PayPal won't necessarily protect you if they find out the item related to e-cigs.
I know a couple of US sellers who accept PayPal but I don't want to mention their names in public because PayPal could technically cut them off.
The whole policy seems stupid and unfair but ebay/PayPal have the right to make their own rules. No one is being forced to use them.