I have been been in the market as a buyer of DVCs on the resale market and here is my take on the agencies. (this is all my opinion)
I think Fidelity (fka GMAC) has the most listings (DVCs listed for sale) but does not do a good job selling them directly. They get the large number of listings as apparently if you call DVC directly and tell them you want to sell your contract - they suggest Fidelity. That said, they charge buyers a $195 "administrative fee" to buy a DVC thru them - a real turn off for buyers vs the other resellers. So they have alot of sellers but few buyers. Not a real market maker.
DVCbyResale (Shontell) and ********** (Jaki/****) both co-operate with Fidelity (meaning they will share Fidelity listings with you... just like a multiple listing service). So for buyers, they are both good places to look for alot of listings. These co-operating listings wont be on their website, and while they will try to sell their own listings before they sell you a Fidelity contract (then they dont have to split the commission) they will sell you a Fidelity if there is one that meets your specific needs.
TSS I think has a lot of buyers looking for DVCs, their fairly priced listings sell way more quickly than anyplace else (I have seen some real attractively priced contracts not move at Fidelity that would not survive one day with TSS's clientele). If I were a seller of DVC, there is no doubt I would list w TSS. On the buy side TSS does not share listings with the other agencies - that limits available inventory for buyers.