POD's on RPGMall.com for the most do not sell very fast. It does however depend a lot on your product and your marketing. For example, the POD for The Complete Guide to Drow that EN Publishing released as POD, did not sell well even with EN World backing them with marketing (though earlier PODs had more success). RPGMall doesn't have a huge customer base that continues to shop there- it's a specialty store mainly just handling fulfillment for you, the publisher. Anyway, once Goodman Games put the product through standard distribution, I'm pretty sure it sold over 1000 copies. I even had customers come into my store asking for the "new" book from Goodman Games, yet on the other side of the store the same book has been sitting there since we opened.
The moral of the story I guess is that small-press products don't sell well. They are mainly just going to see sales from your own grass roots efforts. That said, it doesn't seem like a big deal to doll out $30 to have 10 copies in print at our store and available online if people need them. We even let you take a few of those out of the inventory for your selling at local stores or conventions.
James