The app ChannelReply would solve the issue with managing eBay buyer messages and returns/refunds, for example, but will most likely not be able to help with setting up listings.
If MissionFish allows designated other eBay accounts, that's great!
It looks like your husband's address has been linked to Gmail and set as default.
To check that, go to Settings > Accounts and Import > Send mail as:.
If in the list there is an address different from yours, make your own the default and delete the other(s).
Note that it's nearly impossible to add a different address to your account by mistake, and it ...