Auto-crafted recipes work almost like manual ones, except they are triggered as soon as the required materials are present and the structure is turned on. If you want your structure to work w/o needing power or fuel, just set “activation requires power” to false, leave the “active requires fuel item” blank and set “can toggle activation” to true. It will have an option to turn on and off (you can call it trade and stop trade) w/o requiring anything. You could not set the ability to toggle activation and instead set “auto-activate” to true, to have the structure go active upon placement and never turn off, but it might increase CPU load a little (insignificantly per object, but it could add up on a server).
Auto-crafted recipes work in parallel, but only one item per entry in the default engrams. That’s why a campfire will only cook one piece of meat at a time, regardless of how many stacks are inside it, and why a grill will cook many pieces at once even with a single raw stack (because its “default inventory items” lists cooked meat many times in a row).
To set up buying, create child items for the items you want to be buy-able. You can set their cost by editing the recipe to be X of your currency and the “item quantity”, which controls how many items are returned per crafting operation (in this case, per trade). As child items, they will be eligible in any recipes that use their parent materials (like chitin and keratin are child to a hidden “ChitinOrKeratin” item which you see in most recipes, but some recipes require chitin specifically and neither keratin nor their parent will work).
The problem is selling.
One solution would be to set up a fuel burner structure and make it use items you want to be sell-able as fuel for a very short time, returning your currency (just like wood is converted to charcoal). Unfortunately, it will only return one item per unit of “fuel”, so if you want to exchange (for example) 1 polymer for 5 coins, you’ll have to create a separate “5 coins” item and make it convertible to your basic coin item via another recipe.
Another solution would be to once again use auto-craft recipes. Create a child currency item for each item you want to be sell-able and adjust their recipes and “item quantity”.
If you set the crafting/burning time of each of these recipes to 0.1, it will be very fast without overloading your computer with checks or crafting ticks.
This is a roundabout solution, but I feel like it will work better with the current state of things.