You're missing the step between offering the gubbin at X+15 and selling it at X+15 - namely, the part where someone agrees to buy it. Why weren't you selling a

You're missing the step between offering the gubbin at X+15 and selling it at X+15 - namely, the part where someone agrees to buy it. Why weren't you selling at X+15 yesterday, and why aren't you selling it at X+50 today? Did you just wake up this morning feeling greedier than usual (but not too greedy), or could it be that there is some external constraint on your ability to set prices, and that constraint has changed?