One thing for me is if there are several items up for say 10k and someone puts one up for 9999 I will buy the one below it for 10k. If you are going to undercut someone don't be cheap about it, give a better deal than one dang gil imho : ) I undercut when I need some gil fast or have an odd item I have no need for and just need the space. Checking history is very helpful, if an item has a lot up for sale but only 2 have sold in last month then I might undercut a lot to get rid of the item. I don't do that a lot or with a lot of items, just when it's kind of urgent. But hey, it will always happen so figure out a new strategy instead of expecting other people to make it easy for you.