"I'm not for sale" (a theory)
Sooo, in the interests of finding other reasons for Blaine's outburst to Sam than the SA theory (which I'm still supporting and thinking is right... but I don't want it to be true, hence the having of other theories)...
What if "I'm not for sale" came out of something Sebastian said to him? I mean, coupled with "hey buddy, haven't seen you online"... What if Sebastian's been harassing Blaine online. If he's said something(s) that makes Blaine feel like he's a commodity, just an object for Sebastian to obtain. (Sebastian equates having Blaine with having a Nationals trophy, Blaine is something to be aquired, in a sense. Sebastian objectifies Blaine a lot a lot). Whether or not Sebastian has gone that far with it, maybe it's just that Blaine feels he's being treated like something to be bought or won.
I mean, there are a lot of ways that wouldn't work, because there's still Blaine's reaction to Dave's comment in NoN, and the fact that it's unclear just how "villainous" Sebastian is (he's The Male Santana, apparently, so obviously there's a squishy, angsty side there) (plus no one on Glee is ever evil).
But, you know, what if, is all I'm saying.