to buy or not to buy a used lens on ebay ?

so here is 1 experience with ebay used lens (i am sure that you can find very nice used lens on ebay, i got 1 good at least)

but i will talk here about not so good experience :
so i needed a wide f 2.8 lens
so i was bidding / watching and i got a
24 mm AF 2.8 Nikon,
as amazon states it costs 400$,
now i paid on ebay 120$ (and because i was watching too many i havent noticed that aperture is not working)

then the fun part started, well it has to be serviced, wtf i consider myf((*self an engineer so not service this little mf
so got the screwdrivers 15$
then i broke a screw on the lens, basically i needed to the thing described here : http://shimonmor.com/TEMP/20mmlensrepair.html

so then i got :
dremel 70$
dremel bits 15$
lubricant 10$
pliers kit 10$
and kinda 8 - 10 hours in total
also it required to clean the aperture blades cause they were oiled and this was causing the not closing.

so i got a used lens for 250$ and well lesson learned, read the descriptions more carefully.