If you have two items being sold one with COMMAND and the other with SHOP and they both have the id of the same item. When COMMAND is registered with either sell false or not even with a sell price you cant sell the item with the SHOP type with correct sell price setup.
This is a new bug when i transferred from 7.3.9 to 8.0.0
please fix soon

What server version are you using?

1.15

Issue Closed. Fixed.

I will be looking into this for the next update. Thanks!