So I have an alt in a trade hub doing market trading. It should be able to fund my PvP habit (in 3 weeks I'm about 50 million in the black with 5 million seed money to start). I also have had that character fill up a container with stuff that Jack Dancer can use - 200mm reinforced rolled tungsten plates (50 of them), barrage ammo(30,000), cold-gas arcjet thrusters (50), E5 vampire (50), patterned stasis web (50), etc. This is on top of the 50 million profit. So things seem to be going well.
I had an order in for 20 Rifters - never can have too many Rifters! - and since the price that I was offering was pretty low (120k) the order wasn't getting filled. No problem. I have 18 Rifters in the hanger now, so there's no rush - but if someone wants to sell them to me cheap, I'll take them! I think the highest buy offer was around 180k (far above my 120k offer) with the lowest sell around 230k.
Every morning and evening I check on the markets and adjust my buy and sell offers to be the most competitive (except for this Rifter offer - I just leave it in hopes that somehow, someway it'll get filled). Last night I was doing that and going through my list of about 15 buy offers and 15 sell offers. They are for all sorts of stuff - much of it stuff that I haven't a clue about how it's used. But if it's profitable, then great! Sell me some and I'll sell it to someone else for a higher price and bank the profit.
After going through all the offers I noticed that I had 20 new Rifters in the hanger! YIPPEE! Somehow someone had taken my offer of 120k for them! So I looked at my wallet to be sure of the pricing and all. Oh oh. I DID buy 20 Rifters, but not at the 120k that I thought. I paid 305k for each of them! YOW! Apparently when updating buy prices I selected the item just above Rifter on my list and checked the price and it showed that I needed to change my offer by a few cents to be the highest offer. Whatever it was was selling at 305k, so I (apparently) mistakenly clicked on the line below it (the Rifters!) and changed the buy offer on the Rifters to 305k. Someone, seeing that outrageously high offer, immediately took me up on the deal! I would have done the same too!
So, 20 Rifters at 100k over market value means I paid about 2 million ISK more for them than I should have. In the end, I get 20 more Rifters to outfit and a lesson learned...