from whence poly = selling? I thought poly was "many"?

Greek distinguishes between long and short vowels. Greek πωλη (pōlē) 'sale', πωλέω (pōleō) 'to exchange, sell' (link) and πολύς (polus) 'many' (link). The -y in the English words comes from the Greek -ια (-ia).

Ceci n'est pas un seing.