Customers come in to resize costume jewelry. Sometimes they inherited the ring. Other times, it's a gift from great aunt Martha. But if you're going out to buy such baubles, please keep a few things in mind (I'm focusing on rings here).
1. If you buy one of these silver rings they often have a thick coat of rhodium. Seriously thick. Some factory in China is cranking out this stuff. So once the ring is resized there may be noticeable marks where it was resized because the rhodium has been removed during the sizing process.
2. If you're going up or down very many sizes, there may be trouble. Even two sizes could mean a lot of stress on the structure of the ring.
3. Occasionally these rings are hollow. It's pretty much a guessing game for me and I won't know until I've cut into that ring. Stresssssful.
4. If there is a pattern around the entire band there will be a mark where it has been sized.
5. If there are gems around the entire band, it can't be resized without removing stones (going down) or there will be a section of metal from sizing up.
Some of these issues may not be a big deal to you, but I've seen a few people who bought these types of rings for $250 or more thinking they'll have it resized. When we have to tell them it can't be resized they are very upset. Or they're disappointed that in order to resize there will be a mark left on the ring or some other issue.
So if this sort of jewelry is your thing, pause and consider whether or not it fits. It may not be possible or inexpensive to just take it to a jewelry for resizing.