I would say that most of the 2nd hand items that are sold on SRC are sold at prices below the new prices. Corals wise, it is a grey area. It is easy to say if you buy a coral at $100, then u should sell it at $80. But frankly speaking unless you are decomming your tank, it really doesn't make sense to sell corals at a huge discount from your purchase price especially if it is a healthy specimen unless you really want to get rid of it.
Even at LFSes, you will see different LFS selling corals at widely ranging prices. Some sell "premium" corals at "premium" prices, others sell "premium" corals at reasonable prices. End of the day, it is up to each individual reefer whether he is willing to pay premium or reasonable prices for the premium corals.
As for selling frags, it is a good way of sharing, regardless of whether the end price the buyer has to pay is cheap or expensive. This is especially so for reefers with nano tanks since they can hardly be expected to buy a whole rock of zoas for instance.