Demos are really important, if you can provide them. I liked being able to check out Miriel's jewelry for men, and there have been times I've not bought something prim-heavy because of a lack of a demo.
There's a great-looking set of sculpted pirate boots at Illusion that I'd love to buy if they fit me well enough, but Siyu hasn't had the time to make the demos yet, so I've held off purchasing -- anything that attaches to points like arms and legs needs a close look from me, because I've a rather small avatar by
SL standards. If you sell attachments that are avatar shape-dependent to work right, demos are the way to go.
(And yes, I decided to mention this in hopes that the Summon Siyu spell works.