When were the first civilian-legal AK-47 rifles sold in the US?
I vaguely remember seeing ads for an Egyptian-made version in the late 1970′s or early 1980′s. I must admit that I wasn’t paying much attention to such things earlier. I do recall that when the Chinese copies started coming in, the hoplophobes got nervous because even relatively low-income people could afford them. Ah, those were the days.

I`m not sure if you are talking about converted to semi auto or made as semi auto. I remember in 1964 ( I was 16 then) you could buy "converted" AKs,that were imported from Russia and changed over here in the US. They were`nt very popular because of all the larger calibre surplus rifles that were also available at the time. Golden state arms was one importer that comes to mind.
I was fortunate to get my hands on a Chinese under-folding model back in the early 90′s from a pawn shop. While the previous owner apparently didn’t know what a cleaning kit was, the rifle was in excellent condition. It is such an early model that it is only stamped "Made in China" with no company name. Some I have talked to say it was likely imported sometime in the 70′s. It shoots as well as any full military models from various countries that I fired while in the Marines.
The Egyptian Maadi was available in the early 80′s – 1981 I think. It was the first actual semi-auto AK for sale in the U.S. The Valmet works the same way, but looks different. I believe the Valmets were first available in the late 70′s – perhaps sooner.