You see what you have created for yourself? You should have inform that you dont have that item, but you have another which is similar in all kind of ways, make this known to buyer BEFORE you send out. If he agrees, then no problem la. If he agrees & dare to make noise later, screw him back with the reply stating that he is ok with a different item.

And please, dont keep putting the responsibility on the buyer that he has to check if you have stock or not. Who is the business owner here? Who keep tracks of stock? You or him? He buys from you based on what you have posted, if you no stock, just inform, its not that hard right? His main responsibility is to PAY YOU, not do stock check for you.