I look first at security. If the site looks 'cheesy' then I am not sure that I feel comfortable enough to put in my cc info. Second, if it's an unknown store, I check around for reviews. I also need a site that is easy ro navigate around. I hate a difficult site and never give them a second thought.
In the store I check if they include the maker of the fabric. I hate just looking at a picture of a fabric, but not knowing who made it. I also like a store with good customer service. I recently had something happen with a store and sent them an email which they never answered. They are all over Facebook posting though. I asked why they don't answer their emails and they made a myriad of excuses about being sick etc. But they still havent addressed my email!!! I will never ever buy from that store.
And of course I look at prices. Both for the fabric, and shipping.