I've been thinking about the following usecase: could a retailer/merchant offer clients a "no bitcoin fees" option when paying with bitcoin? i.e. if something costs 1 BTC on the site, the user will pay ONLY 1 BTC, and the merchant would pay the tx fee.
As I understand RBF is a doublespend of the same utxo that, as it has a higher fee, invalidates the original tx simple for being confirmed first.
Or would it be possible for someone else to bump up the fee without having to have access to the utxo owner's private keys (in order to sign the RBF transaction)