I thought the purpose of having the Adjust Price feature was in part to combat the 1 gil underselling bots. Granted it would be a bit time consuming to keep up with all the time. However, I've noticed if I adjust my prices (and I do far more than 1 gil at a time) when stuff stops moving, I find then stuff starts selling again for a while. It slows down, repeat. It's not that difficult and would be a lot less annoying than paying a tax every time I need to adjust my listing price. I think having a tax that worked when you post an item would punish actual players far more than the annoying bots (which SE eventually deals with anyway).