As a former realtor in Westchester & a member of the co-op Board for my building, here is my advice:
1. You need to contact the property management company for this co-op, ask for the representative for this particular building (unless you already know) and find out if the seller is up to date with the monthly maintenance payments. If the seller is up to date - great news. If the seller is not up to date, ask them if any eviction or legal action has been taken against the seller. Get the dates and ask what their next steps will be. If the property manager refuses to tell you anything, tell them they will hear from your attorney - then call your lawyer.
2. Ask your attorney if it's possible to contact the seller's attorney and find out why there's a delay. Your attorney can contact the IRS & find out about any "negotiations".
3. My gut feeling is the seller owes the IRS more money than the co-op is worth and the seller doesn't have it. The seller also has legal fees and I can guarantee the lawyers want a piece of the co-op to pay the legal fees. This leaves the IRS short and who has priority over the cash from the sale of the co-op the lawyers or the IRS.