The sites are listed on GPA's home page. My sense is there is a range from the well-known venues (eBay, Heritage) to those that report very few sales or are in effect no longer operating.
I designed and built Heronext.com a couple of years ago with the help of an accomplished web developer friend. It is not so much a dealer site as a place for comic buyers and sellers of all types to meet and transact. I have not had time to actively promote the site for a while, but a few sellers have liked the unique and useful features and have taken advantage of the free individualized stores. I don't charge a monthly fee and don't take any sales commission. We have probably reported a couple hundred CGC-graded comic sales to GPA over the years. I cannot vouch for the integrity of the sales, other than those from my own inventory, any more than a venue like eBay can. The recent Spirou sales are definitely eye-catching. Anyone here is welcome to PM me about them or anything else regarding the site.