Hi @domsheldon1
Welcome to the Sonos Community! Apologies for the delay.
When selling a speaker, do not transfer ownership - this will indeed move your entire system to a new email, rather than just one speaker.
Instead, please factory reset the speaker before handing it to the new owner. You can see the instructions for doing so on our Factory reset your Sonos product help page. The instructions can vary a bit according to the speaker model, and you should end up with a flashing green light at the end of the process.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein