Just starting, so apologies if this is a completely ridiculous question. Knowing that I've always wanted to learn violin, my family unwittingly bought me a "violin" off of Craigslist. Turns out it's actually a 15" viola (they meant well). My question is if I can still use this to learn violin? Can I just ignore the differences and play violin music/lessons on it anyway? Is the only difference that everything will sound a fifth lower in pitch? Or should I sell it and get a violin? Thanks in advance!

No those are two completely two different instruments that read in two completely two different clefs. You could just play viola I am a little biased but in my opinion viola is a bit better, Everybody plays violin but no one really plays viola.

These two instruments are very close to each other. Therefore, once you play one well you should be able to play the other. But you claim to be a beginner, therefore you will need a teacher. If I were you I would look for a teacher who can handle both instruments, and ask for his/her opinion.

My personal opinion, for whatever it is worth, is to keep the viola and learn to play it. Eventually you will be able to acquire a violin and learn it too. It is good for you to be able to play both instruments. True, you will need to learn the notation in two different clefs. This may be hard in the beginning, but it will be an asset for your general music education. The more you know the better it is for you.