Hi! Thank you for playing the game!
I dont know if is a bug or if you dont know how to play the game, sorry for the inconvenience :/
To sell items all the required items must be near the npcs, prefered in the table.
If you are selling guitars make sure that the guitar has the strings!!
When a item is interactable the crosshair changes the color to green.
If you think that is a bug, can you send a gif or something to see the bug and make a hotfix?
Thank you for the feedback :)