I went ahead and ordered two out-of-print LPs through GEMM. (from 2 different sellers, Some like it Hotter in the UK, and Sonic Recollections in Oregon.)

Apparently, GEMM e-mailed or faxed both sellers immediately, as I received e-mail confirmations from both within what seemed like minutes after placing the order. I received a few more e-mails after that letting me know when the orders were shipped, etc. Both LPs got to me in NY within 8 days from the day I ordered. (just using regular mail.) They were well-packed and in very good condition for used LPs. (One of them was issued in 1959!)

Everything about this was a good experience and I'll definitely order again. A great resource!