Opinions wanted - is this girl interested in me, or am I wasting my time?

I met a girl a couple months ago at a party, she's a student in the same program and faculty as me, just a year behind me. I added her on facebook but never contacted her. Earlier this week she sent me a message because she wanted to buy textbooks from me. We met, and after chatting for a bit i asked her if she'd like to grab dinner, but she said that she just ate so i left it at that. i told her that i'd be studying this weekend at a cafe, and that she was welcome to join me. We met yesterday for a few hours but didn't get any work done, we pretty much talked the entire time. The whole thing ended up being ~2.5 hours and we got along really well I think. But while we were talking about relationships she mentioned that she enjoys being single. I originally thought that she might possibly be interested in a relationship because she was open to meeting up to study (I barely knew her at that point). Later I asked her if she'd like to grab dinner, but she said no because she had to go to a friends place.

Am I right in thinking that she's not interested?

Yeah, you'd be right. You've asked her to dinner twice and both times she declined without offering an alternative date. If she was really keen, she would have said "I can't do it tonight, but when else are you free?"

You've been friendzoned.