Upgrade swap issues

Have an additional upgrade I'm not using. Gonna sell it off cause it's gonna go to waste, but how do I sell the new phone and prevent the new sim from populating with my #info?

Apparently if I have an upgrade for my line, it's tied to my account. If I sell it off and give it to someone else and they activate it, the sim tied to the account(the one in the new phone) will get my number. So when the buyer activates his new phone with his sim, the sim from his new iPhone will have my number deactivating the sim in my phone. Any way to prevent this without calling Att or having the buyer send me the sim? I can remove the new sim before sale as a precursor, but the buyer would prefer me not open the package.

Any ideas?

I can confirm this happens, cause it happened when I used my wife's upgrade.