Best place to buy used ds?

Discogs.

Discogs, eBay and Mercari if I’m searching for a specific CD for my collection.  Beyond that church fairs and yard sales and estate sales.  

I go the garage sales. My last buy was 1100+ CD’s for $150.  I kept about 175, sold about 275 for about $150 and gave the rest to Saint Vincent’s. When all was said and done, I got 175 CD’s for free and I paid for my gas and I had a deduction for tax time. My wife stops into thrift shops and every now and then finds a SACD (for me) and a disk of something she likes.

Oh and if you have a used record store or Vintage Stock has 2 for one sales for every holiday.  They also have everything on their computer system so they can look for something specific for you.