You can actually charge more for selling a smaller amount (with the exception of items that sell for at or almost vendor trash prices). The reason for this is similar to a real world economy. Because it is more work to sell less at a time it costs more, and conversely its easier to sell in bulk so that is cheaper. Everyone wants to move there product quickly in large quantities even at a lower profit so you can charge extra for providing the extra service of listing items at a more consumer friendly quantity. Know your consumer, know what your selling, and you'll sell better. For instance, Logs and Ore normally sell well in multiples of 3 because most synths that use them use 3 at a time. So a stack of 9 will sell better than a stack of 10, and a stack of 3 will sell better than a stack of 2 or 1, and keep in mind that stack of 99 your selling will only make the crafter 33 logs, so they need 3 stacks of 99 to make one stack for themselves.