Personally (for ham radio purposes) I wouldn't normally buy individual
used small components that have a current retail value of under about
$3. I would rather get new ones which have a greater probability of
working (regardless, in my opinion, of the seller's claims regarding
testing) and, in some cases, longer leads. By this thinking around 95%
of parts found in typical equipment are of no value, which simplifies
your calculations.

For more expensive components, something like 10-20% of the new price
is about the average of what I would pay.

Normally unavailable replacement parts and "antique" parts are another
story, of course.

73,
Steve VE3SMA