On the same idea as SKSuser, I saw a thread on here for a DIY nano canister filter that used a tiny submersible pump, found
here
The link to the company that makes the pump is
here. They are a Chinese company, so there might be a language barrier. But its a product i haven't seen anywhere else, and might be popular.
Also, for other ideas for the store -
#1. - 1 ml and 50 ml syringes, for different size tank dosing. My 1 ml syringe is invaluable for dosing my 2.5 and 5.5 nanos, and the 50 ml syringe has become a small spot vacuum.
#2. - I know somebody mentioned the single reflectors, but maybe some sheets of polished aluminum too for people who want to make their own, but don't want to polish metal. (which is a righteous pain in the butt, as I recently learned.)
#3. - Maybe some 100% cotton thread, in black and brown. i recently had a difficult time finding any.
#4. - Cheap, long tweezers.
#5. - various suction cups with attachments for different size lines/hoses. Don't know if this was mentioned previously.
I think your idea is a great one, and i look forward to your store. Good luck!