Let's see. I'd like to add my two cents in here. What I consider when buying:

1. Presentation and postwork in the preview pics. Please note how much postwork you've done. With no note, I don't know if you've postworked out some fundamental flaw, and that makes me goosey. Just a note saying "no postwork except for hair and compositing" does wonders.
2. Lots of feedback or feedback from people who I'm familiar with and respect. Not something you can actually control, but it is something I peek at if I'm borderline. If the item has been up for a while and there's no feedback, I get suspicious. All feedback has to be approved by the vendor before it's displayed, and realistically, no one's going to approve negative feedback.
3. Camera focal length and lighting. For heaven's sake, change the focal length away from the default Poser fisheye thing. As I see it, if someone doesn't know enough about Poser to do that, they don't know enough to have anything I can use. Lighting is similar, to a lesser extent. Here I understand having one promo pic under default lighting, but put most of them under something complementary. There's tons of nice lighting sets for free over at RDNA to chose from if nothing else. And if you're selling textures, WHITE light. WHITE. It's the only way to accurately show the texture.
4. Unique models: Don't bother reinventing the wheel unless you can add something new to it. It's a waste of your time. I mean, really, how many bras does Vicky need? The woman has more lingerie than I do. Do something new and different, and I'm more likely to buy.
5. Versatile models: Particularly if it isn't something unique, make it versatile. Lots of morphs, multiple material zones, easily changed, easily textured for different scenarios.
6. Good texture mapping: Because I do a lot of my own textures, I actually have bought models based on this. It needs to be nicely unwrapped, minimal seamlines, if it's clothing the seamlines should coincide with where seams would be IRL, minimal white space. If I get so frustrated that I have to remap it myself, I'm not going to be a repeat customer.
7. Detailed but not busy clothing textures. Don't just floodfill with a pattern and call it good. I can do that myself. Match seamlines, scale where you need to. Add details. Show this stuff off. It takes time to do.
8. Experience with the vendor. I'm more likely to buy from someone if I've talked with them in the forums or I'm familiar with and fond of their galleries. If you've got tutorials up (shows you know what you're doing), freebies up that are good quality. You'll increase your sales if you pay attention to and get to know your customers. This is particularly important if you're fairly new. With all the scandals that have happened recently, I really need to know you from Adam to feel comfortable with a purchase.
9. Good customer service. I'm a pretty low maintainence customer. If there's a minor problem I'm more likely to just fix it myself than wait for a response. If I can't fix it, I'll send off a polite e-mail. I don't expect an immediate response (some vendors have scared me by answering in the hour. Yow! Guys, 2 am! Get some sleep!), but try to get back to me in the next few days. Be friendly, and be prepared to help with ID10T errors as well. I don't hesitate to buy from vendors with great customer service, and I direct others to them as well.
10. Don't be afraid to give out refunds. Some vendors have gotten very high on my trust list because they were so willing to give out refunds and generally make things right if there was a problem with their package. For instance, one person had accidentally included a couple of poses that weren't theirs. Even though I wasn't a customer, I was VERY impressed when this came to light and she publically asked all of her customers to delete those two poses, gave all of them a full refund, and told them that the rest of the items in the package were still theirs to do with as they pleased. That was way above the call of duty, and I'm very comfortable doing business with merchants like that.
11. Love of the work. The simple fact is, you can't really make this stuff solely to make money off of it. With the market and prices as they are, if you do then your quality will suffer for it. If you make an item because you wanted to make it, and you might as well sell it as well, then it'll come out better for it and it will show.