I wouldn’t recommend PayPal since digital assets are not covered by the buyers protection. As a seller this is ofcourse not a concern to you.

The best way would probably be to use some kind of smart contract. The seller could for example list the amount of coins on a decentralised exchange like Etherdelta and you could then select it from the bid section. However, it could happen that someone else will buy from your seller first. Moreover, I don’t know if something like Etherdelta exists for BTC, as it is only for ETH tokens.

Maybe Byteball could be a solution for you, but I have never used it and it would also require the conversion of BTC.

In the end the problem will always be that you still need an exchange to convert to fiat.