I have purchased an apartment building (with 9 units) in an auction deal.

Within days, I received several calls and emails with offers to buy under PURCHASE CONTRACT AND ESCROW INSTRUCTIONS.

Unless you already have buy & hold investors lined up make sure you have a reasonable inspection period as showing occupied properties can be a real pain unless your buyers only care about the rent & purchase price.

Personally, I've been looking into more fixer-uppers and value-add opportunities where the initial purchase price is lower, and the potential for appreciation is significant.

Purchase price?

Like if they were required to pay 90% of their appraised value to purchase the property at the seller’s option?

Second, having $40k and trying to purchase a multifam in NYC is not going to get you far. 

Tax sales are a balancing act among (1) the need of government to collect property taxes and, if owners will not pay then, then to provide incentives for investors to purchase the tax rights and (2) investors, who need an attractive return for the risk they are taking, and (3) citizen taxpayers, who should not lose their property and all equity because of small debts and perhaps temporary or at least relatively short term financial difficulties.

However, there's no AC and heat there and I have already purchased min splits air conditioners and an electrician has also laid the lines for me.

In my market, agents/investors/wholesalers pitching creative financing get this objection as well.They typically have a 2nd offer (usually much lower purchase price) for the owner to be able to accept now and cash out.