A user purchased service from certain dApp. Although the dApp does not support any function (or UI) to transfer service purchased between users, is it possible that user give it (ownership) away to others by himself without any support of dApp?
Assuming that dApp binds purchased service to Ethereum address and does not allow transferring the service to another address, user has to transfer ownership of the address itself. Though, this will transfer not only this purchased service, but all assets associated with address.
For address backed by private key, user may transfer address ownership by transferring private key, but new owner will not be able to verify that original owner didn't keep a copy of the private key, do this way requires trust between new and old owners.
For address belonging to a wallet smart contract it is usually possible to change wallet owner(s) by calling certain methods on the wallet smart contract.