If you are considering buying a ring, which is not your correct size or in the description advises that the ring can be re-sized by any working jeweller, there are a few things to consider before making that purchase:
Re-sizing Gold, Platinum or Palladium
Most jewellers will re-size 3 sizes up or 3 sizes down anything more would need to be discussed with the jeweller as the design or shape of the ring could be dramatically altered.
Bezel or channel settings can usually be re-sized but if the stones move you will need to have them re-set at extra expense.
Repairing Claw settings
If claws need replacing or re-tipping it usually costs £10-£15 per claw (2008). If you continually wear a claw set ring I suggest that the claws are checked annually. I just had six claws re-tipped at a cost of £85 but if I'd lost my diamonds I'd be looking at a great deal more to repair the ring.
I have consulted 3 separate jewellers on this subject and they all came back with the same response. Silver is difficult to work. Plain bands are ok to re-size, some silver claw set rings can be re-sized but you will usually be charged to re-set the stones too. Anything else is best avoided! Most jewellers charge about the same as if re-sizing gold!
Watch the sizings
I recently brought a size "M" which turned out to be a K & 1/2 fortunately it was gold so could be re-sized at a cost of £22, but was not quite the bargain I thought I was getting! I have started looking for slightly larger rings to avoid this problem.
Some sizes are given as mm, just check this against a ring that fits you well and you will get a good idea if the advertised ring will fit you.
Wide band rings
If buying a wide band ring go 2 or 3 sizes larger than your usual finger size for a good fit
Remember the services of a jeweller do not come cheap to most of us and this should always be considered if buying used/second hand or bought in need of a repair.
Good luck and happy bidding.