You must defeat Sheng Long to stand a chance!
Rakes, my arch enemy.
After reading through the rules, it seemed like there was no rule stating he couldn't.
This isn't a gripe aimed at you in particular, but this is a longstanding peeve of mine. Just because its not explicitly prohibited does not mean it is permitted. Especially the rule that says I can conjure 4 treasure chests at will to open or magically increase my threat count (why can't I? Doesn't say I can't in the rules!).
But seriously now, skill cards are representative of an actual skill or technique a hero knows. Just like I couldn't sell back an education to a university, you can't sell back a skill to the market.