OP: Are you looking for a specific piece of collectible gear? Then, I suggest eBay, or one of the other on-line sites mentioned above (which I have no experience with), or a WTB ad in Bartertown as suggested by C T Jim. The marketplace is efficient – meaning you’ll generally pay the going market rate for a collectible piece of equipment if you use a widely-viewed on-line service.
FWIW, I’m a collector. I’ve always sought a specific piece of equipment. I’ve never found what I sought locally. Almost everything was bought on eBay. In a few cases I’ve purchased direct from a seller an item that was not advertised on eBay. (For example, Vintage Vacuum Audio has their own web site.) I do NOT advocate the unethical practice of finding an item on eBay, and then attempting to cut eBay out of the deal. IMO eBay provides a valuable service for both buyers and sellers, and deserves to be paid for their service.
Or, are you looking to buy anything that is cheap? I can’t help with that. Only you can decide if you enjoy visiting estate sales, garage sales, thrift stores, etc. in the hope of finding a “score” of anything audio related. As tarior pointed out, it helps if you live in a large metro area - where people have historically bought high-end audio gear - if you want to “score” high-end gear locally.