I don't have a cape to sell him haha! Martin probably wouldn't sell his 23 TMA either since he doesn't want the 24 TMA cape on forums anymore so he'd have no upgrade. I'd suggest buying an 11 magic or 12 magic clean Purple Gaia Cape (400m-600m each) and scroll them with 60% or 70% scrolls. If you want 5/5 60%s on them, you'd have a 21 or 22 TMA cape ez.
Thanks for the advice everyone! Unfortunately, however, most of the 11matk capes I've found are ranging 600-700m. And I'm willing to pay a little higher premium if the cape already has the stats I need.