Well Slackware 3.6 (released in 1998) is a bit old. There is nothing wrong with selling some old distro. 10 quid is a bit too much, but what the heck.. it comes in a nice package. So i guess that's ok too.
JUST DON'T SELL SLACKWARE AS DOS
I guess their point of view is: Linux->command line->DOS So there you go "Platform: DOS"