You have a tremendous blind-spot in your value proposition, where "free for free" not only doesn't make anybody money, but it's actually a loser on the store side, since by being everything but their POS system you are basically getting their sales data (at least in the form of some aggregated terms) on top of everything else. This is competitive information that is highly guarded in most cases, and not necessarily cheap besides.

Frankly, taking inventory is the easiest part of this. Many places are fine to do it with just pad and paper.