This has likely been suggested before, but here goes:

I, and I'm sure many others, when looking for cheap rares to frag, find a big shop (or your own shop) and look for low-priced rares that they can then search for and buy several of. Wash. Rinse. Repeat.

This would be so much easier if, when in a shop, you could sort by price. There are also other reasons one would want to sort by price, but that's what brought it to my attention. =P