Ah yes. This is where you have to ask yourself why you have a
seatbelt:

If the purpose of a seatbelt is to avoid getting a ticket for not
having a seatbelt, then a used belt is fine. Once a belt has been worn
in an accident it may look the same but it's not, it has been stressed
and its strength and characteristics have changed. Still, if you want
to save some money in the short run (and live in a thrilling and risky
way) then you might be able to get a belt out of a wreck and have a
local mechanic put it in for you.

If the purpose of a seatbelt is to reduce your injuries in a crash
then the only thing you can safely do is replace the whole thing. You
cannot do this job yourself (or at least it's not advised) because of
the pyrotechnic pretensioner (aka a stick of dynamite) in the pillar
attached to the belt. The dealership (I was told there is no
aftermarket part for liability reasons) is the only place to get it
and they don't repair faulty seatbelts. Installed the whole job cost
me around $620CAD (yes, "UGH!").

I just had to replace the driver's seatbelt in my 95 850 for just the
opposite reason...it retracted and wouldn't spool out again... it just
got shorter and shorter and then was tight. So tight I couldn't
re-adjust the seat (they're attached).

I wish there was a better way. The new belt sure is smooth and new.

Good luck,
blurp