actually its pretty simple supply and demand. There's a fixed number of seats available, which is pretty low considering who gets access first (celebrities, people who bought rights to tickets earlier in the season, season ticket holders, people with partial season ticket plans, the NBA offices, the Celtics family and friends, etc.) - I'd guess there were maybe 1000 tickets available at 11am. People in line at the garden probably got 20% of these almost instantly, others at ticketmaster outlets also got in on these the instant they went online - that leaves maybe 50% or about 500 tickets for the internet. Demand was surely greater than 250 people, I bet 10-20 thousand people minimum were online at 11am all jamming the line trying to get in - I'm sure a few people got their tickets, but they were the exception not the norm. And now these tickets are being turned around for great profit because there is such an inelastic supply and such great demand. People will pay nearly anything for these seats because there are so few. If you really want to get in, you're going to be shelling out top dollar, as others are willing and able to do that.
(Can you tell I teach economics?) =)