Your analysis is flawed. You're using taxes on the full $8,600 award ($4,300), but only comparing that to the $6,500 direct PW cost from Tesla. You have to include the gateway ($1,100) and FULL installation cost (approx $2500) if you buy direct. The $5,500 asking price includes the entire package.
No, people don't want to "make a profit".. we just want to sell them at a reasonable cost that makes sense for a buyer. Without knowing the buyer's effective tax bracket and state income tax (it's only 6% here), it's very hard to make a deal that's fair to both buyers and sellers, or that doesn't carry significant risk for one or the other. Someone who lives in a no-income tax state and has a low effective tax rate can probably get a great deal on one of these (I have four to sell for friends -- 2 sets of 2PWs, so if you're interested, PM me)
Also, anyone can claim the 30% tax credit, even if buying privately.