If Teslas become eligible for $7k federal tax credits, it will have a trickle-down effect on used values; assuming Tesla doesn't adjust new prices up equivalently in the process. Anytime there is a notable price drop whether it be a reduced MSRP, new rebates, new tax credits, etc. it will impact used values.
If a new LR AWD is currently $47k and a similar used 2020 is worth $41k retail, a $7k tax credit would drop the new price down to $40k which means the 2020 retail value (and therefore also the wholesale value) would have to drop sufficiently below that which would then impact the value of 2019s, and so on. The impact would diminish the older you go.
However, if you decide to trade for a new Tesla during this tax credit time then the resale value drop sort of becomes a wash since you are also able to buy the new car for less. Yes, your car is worth less but the car you are buying is also worth less (cheaper to buy).
Whether or not the tax credit returns to Teslas remains to be seen but since there's nothing you can do about it, there's no sense worrying too much about it. It will only be short-term anyway. I think we can expect a price volatility in the EV market for a while.