Hi,

 

I am going through old bins of lego (15-25000 pieces by my estimation)

 

And I have come across a LOT of old castle pieces - primarily from black knights/royal knights from the early 90s.  I remember trying to build one a few years ago and missing about 10% of the pieces so I subbed out different colours etc.

Is it worth it to complete the set to sell it (to get more lego) or just sell it as pieces in bulk.

 

The other old sets like islanders and pirates and city, space police would take too much time.

 

Any help is appreciated, thanks!

Sell it by set, but identify what pieces are missing. If you sell in bulk you'll probably get 50% less. 

Try to sell it by set. If the pieces are cheap enough for you, I would recommend trying to replace them with the correct pieces.

Get the parts.  You will be way ahead in the end if you go that route.  It's more fun that way anyway.