Stacks of items are worth more when sold individually
For example, if you sell a stack of 100 iron arrows by holding ctrl and spamming the left mouse button, they will sell for 100 gold. If, on the other hand, you sell them as a stack by shift clicking they will sell for considerably less. Not only does this make no sense, it can be exploited to make shopkeepers give you free stuff by repeatedly moving stacks of items back and forth in the shop interface.
This bug was also present in vanilla, if I recall correctly.
(RM-1935 from redmine: created on 2014-09-20 by Alexander Bessman, , closed on 2014-09-24 by scrawl .)