It doesn't look like you are getting any answers. Let's review a few basics:

Ford or GM is not impacting if you buy my shares. The same is true for this S Corp. The S Corp is its own entity and was not involved in the sale between 3 humans.

The shareholder that is selling has some sort of basis in the corporation, the same as when buying shares gives you a basis on the initial investment. In the case of the corporation, over the years the shareholder will have activity that updates their investment in the corporation. Now they are selling their share of that investment, so someone will need to determine if that sale prices results in gain or loss. Additionally, anything paid over time likely has an interest component, which is not part of the sale price and not part of the gain or loss, but part of the, "I am willing to be patient about getting my money." That also means each buying shareholder has to track their current basis or investment, and this additional change for each of their positions. And they might have investment interest expense to consider.

It's not clear what adjusting to the books is being done because of the sale, other than, 3 become 2 and the allocations will be affected as passed through on the K-1.

So, there is not really an impact to the 1120 S. There is every impact to the seller's 1040. There is some impact on the buyers' 1040, but mostly, that comes later when they also sell. There is the effect of the financing agreement to all parties involved, most likely.

You have a lot of little pieces to work on, unless you are doing only the 1120 S.