It really depends on the team they're playing and the day of the week.
If the Celts are playing a big market team like the Heat, Lakers, Knicks, etc, it's very easy to sell them at face value and you can probably make a profit.
BUT, vs the lower level teams like the Wizards, Bobcats, etc, it is quite difficult to receive equal value.
Sometimes, another big factor is also the day of the week.
Even if it's a bad team, if the game happens to fall on a Friday, Saturday, Sunday, that makes it easier to sell.
However, if it's a weekday game vs the lottery teams of the NBA, there's a good chance you'll end up getting lower than face value for the ticket, if anything at all.
To make a long story short, I've had to eat some money since I got season tixs in 2010. That's why I then went from a full season to a half season.
Also, the site I usually sell my tickets on is the NBA ticket exchange site.