Are you sure eBay is offering to sell or buy Bitcoin? From curiosity, I searched there, but I couldn't see any option to buy or sell Bitcoin there. Just make sure you aren't going to be deceived by scammers. There is no need for any secret words to buy bitcoin. Secret words or seed phrases are for securing your wallet. No one can offer such words to sell. If you find something like this, it means they are scammers. Who has access if the seed phrase means he has access to the funds? Just try to find a legal way to buy Bitcoin. You may use P2P services to buy Bitcoin with your local fiat.