When it comes to "On-chain transactions" o yung mga nangyayari around Bitcoin's Blockchain wala ka talagang masasabing zero fees kasi yung transaction mo ay palaging may network fees na kasama for the one who will be sending the Bitcoin. However if seller ka ng Bitcoin at talagang gusto mong maka-iwas sa fees I think "off-chain transactions" is the best option para sayo. What are off-chain transactions? Ang mga custodial wallet (Coinbase, Coins.ph) ngayon karamihan kung same custodial wallet yung transaction ang wallet na yun ay walang icha-charge na fees at instant na mapapadala yung BTC sa receiver. For example: Coinbase BTC wallet address to Coinbase BTC wallet address yung transaction na ito ay mangyayari off-chain meaning na wala itong network fees on the sender's side.
Coinbase's Statement about off-chain transactions:
Why can't I see my transaction on the blockchain?
You can view most cryptocurrency transactions on the network's respective blockchain, which records and verifies the legitimacy of transactions. However, when transferring from a Coinbase account to another Coinbase account, the transactions occur off the blockchain. This makes confirmation instantaneous, causes transactions to be fee-less, and allows for micro-transactions as small as one satoshi (0.00000001 bitcoin). For these "off-chain" transactions, you won't be able to view the bitcoin transfer in the blockchain, and there won't be an "Advanced Details" link when you click on the transaction.
Also dapat niyong tandaan na dahil "off-chain transaction" nga sya hindi mo makikita sa blockchain.info or any blockchain explorer website yung transaction mo kasi hindi ito nag-eexist on their part but within the custodial wallet only so sa tingin ko ito yung biggest disadvantage pag dating sa mga off-chain transactions or in internal custodial wallet transactions in general.