One reason is the buyer might not have to put down the entire down-payment initially. I haven't checked the numbers recently but Tesla used to require $50k down on a regular roadster or the entire $250k for a founder's series. With a private sale, the seller could agree to accept a smaller down-payment, with the rest paid closer to delivery. This would allow the buyer to continue to invest or earn the remaining funds. If those who had put down the entire $250k back in 2017 had invested the same amount in TSLA and similar stocks instead, they could've made several million over the past few years. Those who put down the $50k could've earned more than enough to pay off the roadster entirely.