I would think that given the long span of years taken to complete publication (the final part (X) was copyright Elizabeth Sedgwick Child, parts I-IX F J Child) they would have been subscribed in advance, but I do not have documantation for that. As long as copies were available, they would have accepted subscriptions - I doubt individual parts would have been sold, at least not until the publication was complete.
There must have been advertisements in some scholarly publications - the American Folklore Society was founded in 1888, the British Folk-Song Society 1898. Perhaps their journals have some information?
Wonder if anyone on the list has a set? What number ??
Set 471 was a discard from the Princeton Library - pages are brittle, flaking, and many title pages detached. Each part is bound individually, not all the same binding so I guess some were rebound. There is information in these indicating the set was a bequest to Princeton, but I do not have that information available now.

Cheers, Thomas.