From my perspective, you can't fault him. You did have your opportunity to sell it first on eBay right? You did agree to the price you sold to him at right? He does have a right to buy low and sell high right?
Personally, it sounds like too much work to me to search for, collect, list, and sell rocket stuff on eBay. I'm sure he could make more with the time he spends being a pizza delivery person. It sounds like a struggling used car salesman who is turning to anywhere to make a buck, and now will taint his very relationship with what is probably a hobby of his. I wish him the best. I hope it all works out for him. Maybe some day he'll open a rocket shop, retire from used car sales, and sit around drinking Margaritas in Florida wheeling and dealing used rockets. I'd do that!
Give the guy a break. If you're accepting fair price when you sell it, his further business doesn't involve you. If people are ignorant enough to pay his price, let them. Those who know a good price from a bad price will take care of themselves.
I haven't been to the eBay website in probably ten years. I can't stand that site...
But honestly, I don't really care. I'm just bored and browsing the forums for somewhere to insert my two cents. Time to go do something more productive...