"AT Audiogon" "ON Audiogon" Help

Sitting in my chair responding to a fellow Audiogon member I came to a pause..... one finger on the letter "A" the other, on the "O". "What is it?" I ask myself. I need your opinions fellow members. Here's my example
"You can buy cable at Audiogon"
"You can buy cable on Audiogon"
Your votes please.