As best I can tell, whatever you pay to close the deal is your purchase price and is used to figure your capital gains whenever you sell, including overdue taxes, fees, or anything else. For example, it clearly states in publication 523 that if you pay the seller's overdue property tax bill, you can't deduct the property taxes on your tax return, instead you consider the price part of your basis. It seems that logic should equally apply whether the assessment is for taxes, repairs or maintenance. (Consider, for example, if you asked the seller to make certain repairs or else decrease the price. If he did not decrease the price and instead made the repairs, you don't now have to consider the cost of the repairs as excluded from your basis just because they were repairs. Repairs you make after closing are not part of your basis of course.)
You pay whatever it takes to close the deal, and that is your cost basis, including purchase price, attorney and bank application fees, appraisal fee, inspections, and buyer concessions.
I agree that the seller can make certain adjustments to their basis even if you paid for the maintenance, and the seller has to know the difference between repairs, other non-deductible HOA costs, and the improvements. But you don't need to know that.
I also don't understand what capital gains has to do with it now. You will pay capital gains whenever you sell, hopefully at least 2 years from now, and the seller's capital gains are none of your business. If the seller also wants you to pay them extra to cover their own tax, then I hope this condo is worth it, it sounds like a bad deal to me so far. But in the end, any money you pay to the seller is part of your cost basis, even if it is cover their income taxes. (From the seller's point of view, extra money you pay them is part of the purchase price, which further increases their gains and tax, so it's Xeno's paradox, but again, that's not your problem.)
