if they are spining, id take a guess and say the plastic threads are already screwed as u did them too tight in the first place. stupid to be using SCREWS into a PLASTIC thread to hold down a seat that the driver of an automobile has to sit in.
in the event of a crash it would be game over. but hey, thats ebay for u lol. and u probably already know this anyway. best would have been bolts + metal. although they seem to have a hex head? confuzzled.
u will probably need to grab the fuckers with vice grips and twist them out whilst pulling.