Looks to me like a good deal at $50. I'm sure the frame is aluminum, hubs and crank are aluminum. I might pass if the crank rings were steel and riveted togther. But looks in good condition, and a triple crank, if needs no expensive new parts, great bike for getting around town, especially in an urban area or at college where heavily locked bikes still get stolen.
If converted to tubeless, you can still use tubes inside, and I would for improved flat protection.
Check condition of bottom bracket, cogs, brake arm pivots.