There’s not a way to erase an account out of Flyme, but there are eBay policies:
1. Reach out to your seller:
Tell him the item is not functioning properly and ask for the password.
Wait a couple days and if he doesn’t respond, move to the next step.
2. Issue a refund (or a partial refund if you want to keep the phone) and/or contact eBay support by opening a case. Tell them the phone can’t be used without the password and the buyer is not willing to provide it and therefore the phone is likely to be stolen property. Selling stolen goods on eBay is of course strongly prohibited. Also, show them documentation - for example a screenshot that the phone is locked to an account - and that the owner isn’t giving you the password.
3. Either get the password or your money back! :) Also accounts selling stolen property will be banned from eBay.