On a triangular block along 10th Avenue sits the Liberty Inn, one of the city’s last remaining “no-tell motels,” where a room can be booked by the hour.  The building is mostly unassuming, well-maintained and discreet. Its owners expect it to sell in the mid-$20 millions, according to B6 Real Estate, the brokerage marketing the property. Current zoning laws would allow developers to build up to 11,225 square feet on the site—nearly double the size of the current hotel.