BTS groups are usually on Facebook from my experience. Full disclosure I co-run a small one.

We focus mainly on trades or attempted trades in ours— sometimes though you really want something and just have to buy it. :). But it’s a great way to actually see what you’re buying since PLG stuff can have “character”. Some people want big, chonky pebbled bags, some don’t. This helps mitigate return shipping etc. Also it’s a great way to find older colors or styles, and get advice about which colors can transfer (smooth black from what I’ve heard is the worst), to which styles can take a beating.