If I sell such a product, then receive an IP complaint, do I only need to present proof of purchase from a legitimate wholesaler/distributor?
Most likely the complaint will come from the brand of the inventory. I’ve had a company contact me directly and ask me to remove all products. If you don’t they will escalate things and risk your account being suspended. You should be able to present proof of all inventory. It’s wise to stay away from brands that are known to file IP complaints.