A separate value (lets call it Appreciation) for each item which modifies how much it costs at the moment. When you sell the item, it causes the item's value to go down. When you buy the item it causes the value to go up. Over time, Appreciation slowly brings itself back to 0, where it will have no effect on the base price.

This should put a pretty quick stop to players exploiting item loops and funding their entire base off of large amounts of power instead of building processing chains; doing so will cause the Appreciation value to tip the cost of an item loop far enough that it stops being profitable, without affecting normal trading too harshly.

Another value which could be considered is an Appreciation Factor which determines how quickly an item returns to its original value. Things such as ingots and ores would have a very high AF due to being so useful, while things such as Power Armor MK2. would have an extremely low AF due to being a 1-time purchase that isn't exactly useful in bulk.