There is no secondary market for them.
You’re really paying for the retail experience of going to a nice jewellery store etc. That’s where the only value lies.
We always try to sell the rings retail in my pawn shop, even cheaply. Most shops will heavily discount diamond rings if you ask. Not so much for chains etc. If you scrap the rings you get nothing for the stones anyway. Most are too small to worry about.
There are wholesalers who will buy loose stones from you (for not much) but they are really only interested in 1ct plus sizes.
Your best bet is to sell privately on Facebook or eBay, but be realistic because you’re essentially trying to con someone else into paying for the stones that have no resale value, but without the fancy shop and retail experience that came with the initial sale.