Well, exactly. They have had the idea before, as mentioned in the post, but didn't act on it. They probably did a simple cost/benefit analysis and decided it would be more trouble than it's worth.

Once you start taking money and acting like a shop, everything changes. It's not just the implementation, although that would be significant. But suddenly it's your problem if X breaks on IE8 or Y doesn't work with a new version and the developer is on holiday or Z said it would do this but doesn't and the customer demands a refund, etc etc etc. "But I bought it from you!" comes the flood of email. So you'd probably need support staff too.

Well, all of this might be worthwhile if it's a big, rich market. But I doubt it is. How many people actually buy premade WP themes? I would have thought the number is miniscule. Same with plugins. I didn't even realise you could buy WP plugins, actually, until I read that. The idea sounds like a joke. You'd have to be nuts to try and build a business on that.

Anyway I'm sure Automattic thought about it for a while, then realised what a distracting, unremunerative hassle it would be and rightfully canned the idea.