I remember when i used get on this site a few years ago there was a topic about interesting items that people create, so i decided to start this again
This item is a mandolin or really any kind of instrument (this item is specifically for bards only)
this instrument denotes a plus ten on the perform check used to use this instrument
cost:500gold

, the item could have the effect of turning the user into a kleptomaniac, maybe even without the character really noticing it himself. Suddenly, he simply... finds things in his pockets.
500 gp is far too low a price.
An item granting a +10 competence bonus on a skill check should cost 10000 gp, as per Table 7-33 of the DMG.
Limiting your instrument to only being usable by bards does not warrant a factor 20 reduction in price.