Homedude - this is a community, for those of us in need just check out the "PIF" sub forum.
As far as the classies are concerned, "what the market will bear" is a big part of it.
But, mainly, it is because most of the best mods are made in batches by individuals,
and not ordered in bulk from a third party mnf. So, supply is low, demand is high, and
there is a long lead time.
The P18+ from SuperT is in such high demand, a seller could prolly get more than retail
just because buyers would be eager to get one instantly, rather than wait for one to
become available from the vendor and still have to wait for it to ship.
It is a sellers market and, unfortunately, there will always be people trying to take
advantage of this. But, like others said, this allows people to try mods and sell the ones
they don't like, without taking too much of a monetary hit.
For the most part, I think this is a good thing.