If I were to offer re-sizing, how would that work?   I am not allowed to request payment beforehand, am I? Can I charge more for the re-sizing, since it takes time and materials?   If I am not allowed to insist they pay first, what if I re-size, and they don't pay?

For some of my listings, I am able to replicate individual pieces to various sizes, so I use variant listings (eg 4 bracelets at 4 different length choices, however I would be able to supply 4 if someone came along and bought all the different lengths).

For one-of-a-kind stuff, I just let people know that customisations are possible, and to please contact me to discuss options for a custom listing (discuss options here means talk about what I can do, how long it will take and how much it will cost). If people decide to go ahead with a customisation, I end the current listing, make the changes, then relist with all the new pics and specs.