I do not have an accurate success rate to share, each day is a different game. 1 day, I can market 3 domains and sell 2 - another I may market 3 and sell just 1. However I also get interest/sell (some of these same) domains at later stages (some companies can take weeks or even months to get back) - these are not only replies to the previous outbound marketing, but this interest also comes in the form of incoming offers (companies that have not been contacted) either via the parked page or marketplace (Sedo, Afternic, DNS etc.). But I can assure you it's profitable - some days more than others.
Maybe one of these months I will keep an accurate track-record (num. domains purchased, num. domains marketed to end users, num. sales, amounts etc.).
However, and very interesting to note: the reply/sales rate has gone up since I started quoting search volume + more-than-reasonable prices in my opening message.
I am not sure if I should set cheaper pricing for domains that I catch/re-register as the creation date gets set back to zero and maybe these have far less value. But, from what I have seen in the .COM namespace, even dropped domains (whereby the creation date gets set back to zero) can go for thousands if there is perceived value in the keywords/domain. I think the actual keywords that comprise the domain are far more important than age, althrough age can have it's extra bonus/trust factor.