why are some airlines keen to sell?
Airlines that sell their LHR slots are generally those in the "last chance saloon". Airlines who have been unable (or unwilling) to adapt to the new era of competition, who's backers have basically given up on them, and have therefore resorted to selling off the family silver to try and stay afloat. In LHR's case think Pan Am, TWA, Cyprus Airways, Balkan, Malev, BMI - anyone remember them ? They all sold off or leased out LHR slots in their later years, and the rest is history.
Another source of slots is from airlines who have worked out they can make more money from leasing out their slots to another airline, instead of operating their own flights with them. A fairly recent example there would be Qantas who lease out at least two pairs of LHR slots to another airline (can't remember which - either BA or Emirates maybe ?). SAS has recently been selling LHR slots, as did Air Serbia (formerly JAT).