The flip tax is a transfer fee that many new york coops impose on shareholders. In the late seventies and early 80's when many rental buildings converted, huge profits were being made by former renters who bought their units at inside prices and then resold them. Called "flipping" The boards decided to impose the transfer fee and call it a flip tax on sellers to disuade flipping. The link below will explain Flip Tax.
Found in co-ops, it is a fee imposed on the seller under the terms of the co-op bylaws, payable to the co-op, when the apt. is sold. It is usually paid at closing. It can be calculated as a % of gross or net profit (usually) or on a per share basis (but this is more of a transfer fee). There are 2 main reasons for flip taxes. One is to raise money for the co-op, in lieu of raising maintenance charges (which no one likes). They are great cash cows when the market is rising and there are many transfers. The second is to discourage speculation (flipping) of apartments by owners.