Selling co-ops are my niche for over 8 years in Brooklyn and I have never heard of any shareholder only having 1 share, never heard of a 50% resale fee usually its only 1-2% in fees closing, contribution and move out fees and there maybe an additional flip tax fee (this fee does not exist for all co-op buildings) that is usually 1-2% of the resale price.You need to get your hands on the offering plan which you should have and it will contain the by-laws any amendments to the bylaws and the proprietary lease. Either you or a real estate attorney can look it over for you for a small fee to tell you if the board has the right to make such extreme changes.
Better yet, The Business Corporation Law (BCL) is the main New York State law which governs how co-op corporations must operate. The decisions made by courts in cases involving BCL provisions are the case law which interprets the statute. You can find the BCL in libraries which is a great place to start to finding out if your co-op board is following legal procedures.
I hope this answer helped you and if you would like to speak to me more about it feel free to email me or call me directly.