I just wanted to get a feel for where you all are having the best success? For us over at Block and Pixel, it has been TCGplayer. However, that is also where we've put the most effort into it. I know of one business that had major success selling both graded and ungraded singles on eBay. Rather, two separate businesses owned and run by a couple. Links below:
At Block and Pixel we sell some graded slabs on eBay. We've never sold ungraded singles on eBay, though. We want to slowly which to selling them on our own dedicated site. I haven't set us up for that just yet, but it's in the works.
Excited to hear where you all are having success.