I'm sure there are a LOT of excellent modeling supply websites out there with fast customer service and good prices, but to get my business it takes two things I'm not willing to compromise on:
1) A Search function that works! By that I mean one where I can type in a word, phrase or numbers and get results from the entire site. NOT one of these things that has drop down menus and more drop down menus and File Not Found messages, which causes a lot of guessing and frustration and me thinking perhaps I had better waste my time someplace else.
2) Pictures of the products! I don't care if it's a PE fret of 1/700 life rings, I want to see what I'm buying before I spend the money. Especially decals - I need to know if there are enough serial numbers on the sheet to warrant purchase, for example, and measurements for things like diorama bases. If I have to take the extra step of e-mailing for more information, I'll probably shop elsewhere.
If a site has these two things, then I am going to spend some time there and probably end up giving them money. If my order gets filled promptly and correctly, they'll probably have a customer for life even if they cost a little more. If not ...