And also Grover was sometimes a salesman himself to that blue guy and to Kermit.
Cookie Monster would probably have been a good one for Lefty to sell to (since he can be a little naive at times) but the problem would be that little would interest CM unless it's something to eat (remember on that Guy Smiley game show how CM wasn't even happy about winning a jet plane because 'No can eat jet plane!'). Plus you'd also have the problem of Frank Oz having to work both CM and Lefty (as with Lefty & Grover and Lefty & Bert).
Now Oscar would be a great prospect for Lefty since he LIKES junk! However, the sketches then would lack a comedic and interesting element. All it would be is:
Lefty: Hey bud, you wanna buy this here thingamajig? It's only a nickel.
Oscar: Hey that looks beautiful. I'll take it. Here you go.
Lefty: And here you are, thanks.