A skillcape can be bought by any player that 'achieves' level 99 in a certain skill (like Herblore or Magic) or minigame activity (like Dungeoneering) or by completing all Quests.
Trimming the Capes[]
The skillcapes you get from skills all can be trimmed by 'achieving' a second 99 (usually Fletching or Cooking)
Skillcapes from activities can be trimmed by getting level 120, which anyone can do in no time.
The Questcape cannot be trimmed.
Offering someone to trim their capes will be classified as Item Scamming, and will result in a ban.