A cap would be impossible to enforce; ICANN can't monitor someone forking over the cap + a briefcase of cash between two buyers god only knows where.

And super-valuable domains are minuscule portion of overall domain sales, it simply wouldn't be worthwhile.

And it need not be a "sale of a domain" at all; it can be the sale of "ww.com corp", which owns the domain, without any domain transfer at all.