When adding items to vendors that are in stacks.
If the item you are adding tries to stack with one already on the vendor and fails, bouncing off. It will cause the entire container to lose its 'not for sale' tag and become default price.

The same thing has happened for ages with oil cloths. If an oil cloth bounces off one already in the box/bag, the box becomes default price.
The box, and everything in it, can be bought for a few gp.

Now the same thing is happening with bandys, petals, arrows, everything else you ever sold in a stack!

Although not a stackable it also happens with savage kin paint ...
Hope they fix it soon!!

Thank you Petra for pointing this out, as a merchant it really is discouraging ...
Maybe post this in the Gameplay Feedback forum too where there is
a thread on stacking issues that Serado requested feedback on stacking issues ...