Did it once while I was under 10k. Extremely time-consuming. Since I passed the 10k mark, I know that alterations to my inventory can only be minor and besides, each BL member knows that if they order the full quantity of a certain piece one store has, they may end up a piece or two short. I had one larger order recently, a fellow reseller emptied my inventory on some parts I had on cheap, I was missing 6 of one piece, 5 of other, 3 of another yet and then 1x and 1x. Completely negligible compared to the size of the order and of course he didn't mind in the slightest, it happens to all of us...
The important thing is you pack your orders well (ziplock bags preferably), and that you are honest about your pieces (used/new) and that you ship fast if they need them urgently, all the rest is secondary...