However, wheels stack to 3, meaning you're using 4 slots to gain 1 repair in the future, compared to Toolboxes, which also stack to 3, but only use 2 slots, meaning you're using 2 slots to gain 2 repair in the future. While some events can only use Wheels, they are relatively rare, meaning you will likely use that wheel to repair a slot. For example, if you have 4 Wagon Wheels and 5 Toolboxes, it is almost certainly better to discard the wheel. Also, consider the "opportunity cost" of discarding that item.
The guaranteed haggle, not to mention the drastically improved prices they get are incredibly valuable