Harvesting? lol I call it harvesting when I use one doll for parts to fix another doll that is in better condition.
In the asian ball jointed doll hobby, it's called splitting. Mostly because the doll is bought between any number for people, each buying a part of the body. One takes the head, one takes the body/ If it's a fantasy doll like Soom Monthly dolls, they come with fantasy animal parts like wings, a second sleepy head clothing and tails.
Some people buy the doll as a fullset... with clothing, shoes and accessories, they then split it up and sell everything separately.
I don't know if eBay has a term, since a lot of people buy big lots of items, keep what they want and sell the rest.