It doesn’t matter when you sell the used car.
If you want to make sure you will be entitled to the full credit for buying both cars in 2021, you will need to make sure that your tax liability exceeds the amount of the credit. Your tax liability is line 22 form 1040. Another way of saying it is, if you had $10,000 of withholding on your W-2, and received a $1000 refund, then your tax liability was $9000. If you had $10,000 of withholding on your W-2 and you owed $1000, then your tax liability was $11,000.
If you expect your tax liability to be less than $15,000, there are one or two things you could do to increase your tax liability for 2021. Then the credit would offset the extra tax. For example, if you converted some IRA funds to a Roth IRA, that creates taxable income. If you offset the tax with the EV credit, then you essentially convert the IRA for free and then withdraw money in the future tax-free since it is a Roth. You might also sell some in investments to realize your long-term capital gains, and then offset the gains with the EV credit. Just remember to wait at least 30 days before re-purchasing the same or similar investments.
