Skill Sellback is now available to try out in the CMA. The option will only appear if you have at least 60 GS in the character's home chapter, and the option hasn't been used already this month.
To access this, view a skill you have purchased. The Skill Sellback section (if available) will either show you a bright red "Remove purchase" button or describe why the skill can't be removed. If you want to sell back spell slots, links to the individual skills have been added within the spell pyramid interface.
If you sell back a skill by mistake, this can be undone using the usual Undo option, which will also refund the GS.
There's not yet any special options related to selling back Weapon Master (and related) skills, but most weapon masters probably also know some ranged or style skills and shouldn't have a problem meeting their prerequisites in practice. I'll see about adding something for this at a later date.