Yes you will both get banned. What people do though is when they sell EEPROMs they typically come from an xbox thats had its TSOP flashed, when they flash the TSOP they will take a backup of the EEPROM to sell, then flash a banned EEPROM onto that xbox. So the EEPROM they sell wont have another copy of itself anywhere, so when they sell it the buyer need not worry.
Though there are some people who leave the EEPROM on the TSOP flashed xbox, and even others who sell the same EEPROM many times over, but once the scammers are found out they are quickly exposed to everyone so you know not to deal with them.