You will need Ethereum coins (ETH) not just to pay but to exchange your smart contract data as well. That means, even if you are the seller and it’s you who is going to get paid, you will need some Ethereum coins to sign or close a contract. The reason is, whenever you deal with any cryptocurrency, you pay a little bit to send any data within the blockchain. We recommend to have at least 0.1 ETH at your account.
Where to buy ETH (just a few websites out of dozens):ChangellyBitfinex
(using this link, you’ll have a 10% discount on the Bitfinex fee)BitPandaMagnetic Exchange