The SLOT system is awesome. It allows people to preview what they're buying before they buy, which is especially useful for horse eggs. Problem is, if you don't have a full chest of the item (and a hopper above feeding into it), the whole system breaks after someone buys the first thing in that chest. Is there some way that a delimiter of some kind could be added to give the option of cycling through the merch?
What I have done in the past, is to fill the 'sell' chest full of dirt, except for the sell slot in which you will have your item. Then hopper a chest into your sell chest and fill it with the product you are selling. (Yes, I am assuming that you are not selling dirt, the dirt is just a 'filler' to prevent the items you want sold from going anywhere else except in your sell slot) selling by slots are great (as you need to click it twice to confirm what it is you are getting). Preview signs are fantastic as well, so players can mouse over the item!
That's what I'm currently doing, yeah. It's just annoying to need two chests and a hopper to get functionality out of the system.
