i see part of the issue, he want to buy two different rough diamonds for 2 different prices, one for 9 (want of 2) and the other at a 11 (want of 1) and both are listed as potential low.
Once he sells out of one stack, he will not buy for the other stack. Saying that the first stack is sold out. Well, at least i got 18 gears, can't put all 3 diamonds for sale.