I guess even Chase and Wells Fargo have their % limits on ownership, my coop is about 65% sold and buyers have gotten mortgages within the past 2 years from one or the other.
It's important to read through your proprietary lease - are their any requirements regarding sales by the sponsor, either in a % of units or within any timeframe? Who is in control of the board? Sometimes, the sponsor must relinquish control after X years. You say that 50% of the units are sold, is that based on number of shares or the number of units? The # of shares may count towards these percentages more than unit counts. Maintenance fees, voting, assessments are all based on shares. Has the sponsor issued annual amendments to the prospectus? The sponsor is required by state law to do so, IF he's marketing units (which apparently he's not). As a unit owner, you're generally entitled to read the minutes from board meetings and annual meetings and you should also be receiving annual audited financial statements. Contact the property manager to allow you to review the minutes (they do not necessarily have to provide you with copies, you may have to read them at their office). If what is happening or not happening is in conflict with the proprietary lease, or the board or property manager refuses access to the minutes, you can contact the NYS Attorney General's Office for assistance.
There are also private mortgage lenders that do not work within FannieMae/FreddieMac federal mortgage restrictions. Perhaps you could offer to buy down the interest rate or reduce the price slightly to a buyer to compensate for a slightly higher interest rate from a private mortgage broker.