The biggest problem I see is that we're not sure yet that Teslas will get a tax credit.
While anything is possible that assumption has very little evidence to support it. Both bills that have been introduced include language that specifically addresses the two US car makers that are currently excluded. Aside from other GREEN aspects of the bill, what you suggest would negate an entire section IV of the GREEN act and nullify the entire Senate bill. There is low probability that either happens. There are too many arguments for passage of an all inclusive tax credit Bill of some sort but the real question is when.
Currently we have to get through the COVID Relief bill and while it is moving along with a deadline to be signed by the President, no Tax Credit for EV Bill will even be discussed. Second, the House will be focused on Infrastructure Bill next. The Senate will continue with their task of Biden appointments approval. If we thought the COVID relief Bill was difficult, wait until you see the civil war with the D's on Infrastructure. I have a feeling that the GREEN HR848 could get added into the Infrastructure bill. Then when it gest passed in the House, the Senate might want to amend it to include The Electric CARS Act of 2021 parts that nullify the expiration based on success of the manufacturer and replace it with the expiration based on a calendar date. Including these EV and GREEN bills into Infrastructure would be a typical way the Congress would save time and be able to hide the clauses in a multi thousands of pages to get it past those who would be opposed to this spending bill. It would also save the Senate one Reconciliation vote for something else and pass it with 51 votes rather than 60.
Anyway, as to the sale of used EV read section 36B of Title IV of the GREEN Act HR848 where sale of a Used EV tax credit is added. I haven't studied this part for details yet but it is something anyone who wishes to sell a used EV should understand.
The other thing that is important to know and not forget, The Electric CARS Act of 2021 changes the start date of when EV delivery is allowed which is Jan 1, 2021, while the House Bill HR848 sets this to the date of signing into Law.
Overall, I am pretty sure some form of EV Tax credit will get passed for all EV's, including Tesla. To suggest otherwise really has no history in Congressional law making. It would be like saying everyone gets a Tax credit except ChadS. They just don't do that.
So, we have evidence the Congress will at some time be voting on an amendment to the Original EV Tax Credit law, but we just don't know when and what the details will be.