Please allow stacks of items to be posted in the exchange with a "per item" price to allow buyers to purchase only part of a stack. Make the "per item" price an option when the seller creates the listing.
Economic Side Effects:
My observations have been that people always list new items on the exchange as slightly cheaper than the pre-existing items, and that people always buy the cheapest item listed. During time periods where there are lots of sellers but few buyers, the prices of items drop (due to every seller trying to be the cheapest). When their are more buyers than sellers, the prices of items increase (due to all of the cheap items being bought, increasing the price that someone can list the "cheapest" items). By allowing someone to offer "10 Widgets at 100 Energy Credits per widget", I believe the market for stackable items would see a rapid influx of new goods and a long term overall decline in prices.
Since there is currently no fee on auctions, this should have no affect on the overall supply of energy credits within the game.
I've recently been grinding diplomacy, and as I result I've also ended up going into business selling Data Samples. On average, I'm picking up ~10 data samples per diplomacy mission. In general Data Samples are sold with a stack size of 1. Since players are limited to only having 20 items on the exchange at a time, I find myself constantly running back to the exchange to list new items. First thing I do when I get up in the morning is hit the exchange; then I check it again after breakfast and again as soon as I get home from work, and then anytime I'm near a space station while playing. I'm feeling more like a Ferengi than a Star Fleet Envoy, and I'm still not able to list items fast enough to avoid having a backlog of data samples in my inventory.