Tell them you are an artist selling artwork, but do not overestimate how much you are selling. Eventually they will send you a certificate to post wherever you regularly sell art. When you sell art you must collect sales tax, which is sent at year's end to the State, along with a sales tax return. However, you do not have to pay sales tax on any of your art supplies. Before the purchases are rung up, announce that they are for resale and demand to fill out a resale card.
Stores keep regular customer's resale numbers on file.
Tools and equipment are still taxed, but sales tax on all consumable supplies can be avoided. It adds up. Other than having a friend put your art up on their site, easy ways for local artists to get wired are to join ProArts in the East Bay or SF Open Studio, or look for nonprofit art sites in your area. Both have web sites for which you pay for the space and to have your slides scanned. I believe SF Arts Commission is starting a project with its slide registry, and all other slide registries will be digitized soon.