so I experienced this on multiple occasions. If I want to make auctions on the auction house, the buyout price changes autmatically.
Example:
I put something in for buyout of 99g, then I look up my auctions and it turns out the buyout price was set to 1980g (happend to me exactly that way).
Is this a know issue? I'm 100% certain I typed in 99g. It happend multiple times. I also don't have any addons that change the auction house interface or something.
You probably entered price per item when selling 20 items. You should change it to price per stack. There is a setting for that above the price you enter.