Real time thrift shopping. Find an object in store, list it, walk around and keep listing items. Get paid what you're asking, then check out and ship it to buyer when you get home. Something like that. Or have a list of items people are requesting that they can't get where they're at or are hard to get...

Definite

I can see an online marketplace/listing type relay service. Make it a paid relay (one time, sub, or per note) and you can cut out the spam. Would be great to have a client that allows you to toggle bundled relays with a single click. For example, turn off all your normal social relays and turn on your merchant relays. (Or just make an alt account)