Also there is
http://www.musicstack.com , which is very good as well. Their layout is way more user-friendly than gemm, and they also screen their sellers before allowing them to list on their site. Our store lists on both of them.
Just be sure to keep an eye on the seller rating; there are a lot of very good people on both those sites, but there are also those who charge ridiculous prices, have shoddy fill rates, slow shipping, general ripoff tendencies.
These sites are great though, and without them our store would sink.
Great for comparing prices too!