Legality is very murky, but the acceptability among the custom "community" would probably be dictated by the agreement between the artist and buyer. Sometimes sculpts are made with the understanding that they will be recast, sometimes they are made with the understanding that they won't be.
Well Rainman's own contributions would likely be limited to sculpts, accessories, and painting. If he re-used the sculpts, then it seems like the desirability would drop significantly. But then again, I saw a figure on eBay recently with a small run Yunsil outfit and Rainman paint (not sculpt), that went for over $1,500.