I've read some of his most major works (technofeudalism, another now and his account of the Greek debt crisis) back when I was still a leftist and looking back the amount of bs he spouses is astonishing, selling more snake oil than Henry George and his "one simple trick".

Capitalism is literally a system of commodity production (production for exchange) and due to the nature of commodities markets get created. "The wealth of those societies in which the capitalist mode of production prevails, presents itself as “an immense accumulation of commodities", its unit being a single commodity." is the very first sentence in Capital by Karl Marx.

What Varoufakis is saying here is that if stock market or private ownership transfers get restricted, suddenly it's not capitalism anymore which is not true at all, it doesn't change why things are produced or what they lead to and also indirectly calls state capitalism (where the ownership is (largely) nationalized) as being anti-capitalist.