I was selling mushrooms, and after I sold a stack it said admin shop ran out of money…
Since when is it possible for it to run out of money? 
It happens quite often. Especially when selling gold xD
There is a recession… so I guess it was bound to happen
just buy some stuff from it then you can sell stuff again
Well thats pretty unexpeted. I often use the admin shop because player shops often have not enough money or are out of stock. 
I did not know that could happen! Perhaps it was on purpose to limit the abuse of selling cheap stock? SpecialK is likely the one who set that up, and he is in China.