I owner financed a condo. The buyer did not pay property tax or HOA fees for a year. I paid them. Can I deduct the payments?

No. HOA fees are not a deductible expense on a primary residence, regardless of who owns it.

How are you reporting this sale - on the installment basis, all in the year of sale, or is it tax-exempt due to the $250k/$500k exclusion?

It is tax-exempt due to the $250k/$500k exclusion

As it is tax exempt, even if you added the HOA fees to the cost of the property as a cost of sale it would only serve to decrease the already untaxed gain, resulting in no tax benefit.

Yes for Property tax.

http://homeguides.sfgate.com/gets-tax-writeoff-owner-holds-deed-mortgage-61734.html