I think this is actually a really smart idea for an internet store. There are a lot of people who have a difficult time finding the grids in their area, and end up paying a lot of money to have them shipped, or spend days and lots of gas driving all over town. I had a lot of problems finding coroplast, and when I did find it, I could ONLY buy it in the full 4'x8' sheets. (They would cut it, for an extra charge, but I had to pay for the full sheet.) Not a great value when you only need a smaller piece, or if you have a small car and can't transport the full sheet.
At least by ordering from this store, you know you'll get exactly what you need for your selected cage. Plus, it gives people who might otherwise get a little pet store cage another option, even if they're not super-creative.
If you think this is a silly money-making venture, you should spend more time on Etsy. There are plenty of things there that are just crazy. LOL.