if you add it at 0 silks, even when having 0 silks on your account, the system will trigger an if
Code:
if(acc.silks<=0){
dont even bother to start the transaction
}
that way, the transaction wont take place even if the value is 0
you can make it 0 and give away some silks, like 10-20 if anything is gonna be free
as for adding it on the the shop(NPC), try playing around with shop txts