On this one, I'm trying to think it through. There may be a way to achieve payments without being a payment processor, and instead serving as an escrow agent, especially if the store is more of a "middleman". I'd be curious to hear what others think. If instituted this way, then price is determined at time of sale. I need to really think about it and draw something up. A lot depends on the kind of store - to be a retail store like dogestore, you would have to interact with suppliers. But if it were more of a "user sells his stuff" store, performing escrow might work well, as no fiat would ever have to be exchanged.