There are several buyers/sellers who trade spheres for objects.
Apart from that you can only find people who are also using the underground nearby (in real life I mean). It uses the DS's local wireless connection to see other DS's nearby.
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.