You, as the buyer, are paying $326K. That's your purchase price, and your cost basis for when you eventually sell. Period.
I don't know why the seller's tax situation is any of your business, or why you are getting involved. They can get their own tax expert to advise them. Unless this is a relative or personal friend, the seller is likely to get very angry if you give them advice from the internet and it turns out to be wrong if they are audited. I would strongly suggest you stay away from giving tax advice to the seller.
I generally agree that the seller can treat $326K as the sales price, and can also treat the portion of the HOA assessment that is improvements, as an adjustment to their cost basis. Determining what part of the assessment is dues (not deductible or a basis adjustment), repairs (not deductible or a basis adjustment) improvements (a basis adjustment) and arrearage fees (not deductible or a basis adjustment) is best left to the HOA's accountant or treasurer. The HOA should probably provide a detailed breakdown of the assessment (to all the owners, as a matter of routine). But this has nothing to do with you and I suggest you stay away from giving the seller tax advice.
