Basically, any form of art is. This includes:

Visual Art
Graphic Design
Poetry
Prose
etc. (Along those lines)

What you CANNOT do is post something that violates Gaia's TOS.

You may also set up a shop for the guild members to buy from-- but the gold goes towards SWC. (Like a donation that you get something cool in return for)

But remember, there probably isn't rich gaians on here, so keep prices reasonable? If they're hiked up too high, nobody is gonna be able to afford anything from you!