I am not a lawyer so don't take this as legal advice, but coming from the telecom industry myself in a respect, I would say that this isn't an eBay rules thing but a legal contract issue.
Unless there are provisions in the provider's agreements for a transfer of a contract, that selling a phone on-contract would be a violation of contract. The phone during the contract is deemed as part of the service and not a product and the contract is bound to a person and not that device. Therefore, if there is remaining owed on the account as according to that contract, selling the device would be selling something that they really don't own yet, thus would be selling something that isn't theirs to sell.
Given this it'd be a very gray area and depends on the subject contract, I'd hazard that due to the legal questionability of such a sale and hazard to say that unless the seller is taking care of all of the contractual elements (and more importantly: is able to take care of this — not all providers permit transfer) that it'd be if not an eBay policy violation, it'd be a violation of contract and potential theft of the device from the provider.
