two things:
the passionate fans who are season ticket holders purchase the tickets with hopes of making it to the finals and witnessing a title. so most wouldnt want to sell.
secondly, those who have courtside seats, most likely are extremely financially secure, where the amouth they could get wouldnt matter too much to them.
im a celts season ticket holder and i would never sell mine. i have had plenty of opportunities this post season. i understand it is different for boston fans, as it has been over 20 yrs since our last title, whereas the lakers have doen a few yrs ago.