For me, if you're a store (online or not) that buys direct from a supplier you should stick with cover price. You shouldn't start going through your back issues and adjusting - that's the charm of a comic store - finding gems at cover price.
Resellers can charge whatever they want as they're matching the market trends, but if you bought in bulk at cost price from the supplier - sell it at cover.
Just my thoughts, would prob think differently if I owned a store...
I would push again for one set at listed price and money back for the rest.