What's the most graceful, facesaving way to reject invitations to product "parties"?

You know those people at work who always want you to come to their candle, gadget, kitchenware, body care product, sex toy or [insert useless product here] parties—what’s the best way to say “no” and still maintain a friendly work relationship? I don’t want to ruffle feathers or look unsupportive, but I also don’t use those products and wouldn’t buy them.

(Apologies in advance if this question’s already been asked – I’m at work and don’t have time to go on a wild Fluther chase.)