This is a popular line of attack since it's always at least tangentially relevant. Unless the critic is maintaining some kind of absolute Luddite existence in a shack built with a homemade handsaw, he'll be tangled in some way in the tentacles of the modern economy. The alleged hypocrisy is so sordid in its immediacy that it can derail absolutely any line of critique.
The Marxist, here, has a distinct advantage over the Liberal. Contrast their positions.
The Liberal argues that the evils of Capitalism can be mitigated through some combination of self-and-state regulation. Managers can practice ethical management; consumers can be conscientious consumers; and if all else fails, we can pass rules and regulations to impose some degree of order and humanity onto the system. Writ large, all of this is accomplished through the accumulation of individual decisions - in the board room, at the cash register, and at the polls.
In other words, the right-wing critic has a point. The Liberal can agitate for change and decry the evils of Capitalism all he likes - but to the extent that he is not fully exercising his franchise as a manager, consumer and voter, he is not playing by the rules of his own game.
The Marxist is in a different situation. He denies those rules from the very start and refuses to play by them.
The economy is not dictated by the managerial discretion, consumer choice and democratic will of individual actors - not according to the Marxist. The situation is more complicated than that. And we can elaborate on those details if one likes, but here it's enough to simply point out that the Marxist does not believe that his personal decision to buy or not buy an iPod is what makes Capitalism work. Nor does he necessarily believe that Capitalism was even necessary for his iPhone to exist in the first place.
The Marxist may be mistaken in all of this, but as I wrote about last time, hypocrisy doesn't exist where the premise is in dispute. Unless the Marxist actually agrees with you that his personal consumer decisions matter, he is not acting in bad faith by proceeding as if they do not - he is merely, at the worst, incorrect.
Which is all to say that legitimacy of the Marxist's position has nothing to do with whether he owns consumer electronics or whatever, and everything to do with the substantive merits of his economic critique. The apologist for Capitalism can't escape that question with the Marxist as he can, to Liberalism's shame, with the Liberal.