Incoming student arriving next week: What are the best local channels for buying a used PC or 2nd-hand parts for building my own?

Hi, I’m a new PhD student in Computer Science and Informatics, and I am arriving in Michigen next week, and this will be my first time in the US. I need to get a PC set up ASAP for research (deep learning, NLP) and daily use (also AAA gaming). But as a student on a budget, I’m focusing on cost-effective options. I’m considering either buying a used PC directly or building one with 2nd-hand parts. I have basic PC knowledge and I know the target configuration, but I’m not familiar with local resources. Could you please share your recommendations to me? Thank you guys a lot!