If you cuold post a sign in a computer repair shop

You could write in forest green "Recycle your old, outdated, or broken computer here for FREE" with a arrow and recycle logo under it. If you want to make your sign look really professional, get it printed on one of those corrugated plastic yard signs and tie it on a wall or wherever he want's it. Something like this:

They cost about $10 each, but you should be able to pay that off within the first few computers you get from posting it.
Custom Recycling Reminder Message Sign, SKU: S-3565