jeff chimed in before me. if I were in your shoes, I would have probably leaned on Craigslist and the area subreddit (the latter is very hit or miss, but I could see something niche like records doing really well.) I'm not a vinyl guy, but I'm sure there are a TON of niche forums out there worth looking at as well.

I really wish you could apply multi-armed bandit approaches to physical problems like these more easily; obviously its hard to do A/B testing when you only have one actual supply.