All other factors held constant, resale prices will drop when the tax credits become a reality. How could they not? It’s simple economics. Suppose there are buyers willing to pay 10K above new just to get their car now. Drop the price of the new car by 8K due to the tax credit and now many of these buyers (the ones who qualify for the credit) would need to justify spending 18K more than the price of a new vehicle. This will cause some to decide to wait for the new car, forcing some resellers to drop the price of their used car. These market forces remain the same regardless of chip shortages, etc.
Of course the other factors will not be constant. Tesla could continue to raise their prices in response to the credit, which seems likely. I believe that some of the increases we’ve seen are already in anticipation of these credits, though there could be more. They will also ramp up production in Austin soon, boosting the supply of available new cars and shortening the wait time. But the market force of the 8K credit will still be there, affecting the price of both new and used cars (which don’t qualify for a credit yet). This is an interesting time where someone can buy a car and then turn around and sell it for 10K. It won’t last. Eventually the adage about a car losing 10% of its value after it is driven off the lot will be true again.