Back-of-the-napkin math/corroborating the comment on the calculator page: for full markup, you can sell 52 tier 5s, or 10.4 tier 6s, or 2.08 tier 7s. If you'll take the half markup, you can sell 144 tier 5s, 28.8 tier 6s, or 5.76 tier 7s. So if you're, say, stockpiling Night-Whispers to sell to the Rat Market, you'll only get to sell ~58 of them each time it shows up.