Just want to point out, I'm not an expert-just voicing my thoughts based on my little bit of experience in custom runs and what I've seen in others. Also, I apologize if the below is a little long winded.
But to your question: It’s my understanding that usually the artist sets as a limited edition, at least for the higher end sculptors. That's why I mentioned previously that more and more customs sculpts coming with a numbered COA and, at times, a number or marking branded inside the sculpt depending on the artist. And like any other higher end artwork not exclusive to 1/6 figures, the fact that it’s sculpted by *insert popular artist* and marketed as a limited item makes it more valuable to the ppl buying. Usually the higher price on these custom sculpts correlates to the the limited nature.
For example: there was an Indiana Jones headsculpt that was limited to a select group of ppl (invite only I believe) that was about $800 if I remember correctly. So, imagine if you just paid $800 for a very limited item that was only meant for say 10 ppl, which was why the cost was so high, and then the artists out of nowhere decides to sell more. Most would probably raise an eyebrow at the very least if not their voice, b/c the whole point of them paying that high of a price was due to the limited nature and now the artists is going to make more?
How dare they? If that was me, I know I would question why I had to pay so much if more were going to be offered in the end and I'm sure I'm not the only one. If all of a sudden 50 were now going to be offered when it was originally supposed to be only 10, I would not expect to pay the $800, but significantly less. I would want a refund and/or probably never buy from that artist again. Doing something like that lacks integrity and I don't think I'd be the only one who'd feel slighted if that were to happen.
The customers may not be looking for resell out right, but there's definitely a psychology behind it. Most humans like to know what they have is valuable (especially with artistic/creative media) and selling something as a limited run adds to it. Scarcity helps create the value. I've had this happen to me before with parts on eBay where a seller will have a certain part at a higher price than I'd like to pay with the quantity is set to "last one!" in red lettering; they're the only one selling it currently, so I buy it only to find that the item gets relisted again
Another example: lower edition sizes usually matter to statue collectors and it’s usually a flex if you got a low number out of the production run and almost better yet, an Artist Proof. And related to 1/6: quite a few ppl have suggested rooted versions of InArt figures to include edition numbers b/c of how
premium they're supposed to be. And I believe InArt will be required by the licensor for Joaquin Phoenix's Joker to include an edition number b/c they also perceive that adding one creates more value and only allow the license for "high end" collectibles. It's not about resell value necessarily, but more about perceived value.
I, myself, admit that it does feel nice when I get a statue or other items with a low number and sort of "eh, that's ok I guess" when I don't