A lot of sellers of fine jewellery offer a resize service on rings....I find it very handy and go back to these sellers regularly.

I also do not particularly like sets, but I do like to be able to buy each part of a set as a separate item.  Some clothes do not need a pendant/necklace but do need earrings or a ring or a bracelet.  A lot of sellers will list each item individually but mention in the listing that there are matching pieces listed separately.