Three (!) years ago I bought a second-hand lens from Henry’s in Canada. Sadly both front and rear elements where scuffed to hell. Not cosmetic issues – the lens produced unusable images – the central sections of all shots were muddy with awful ghosting and light steaks. Not arty, just nasty.
After having waited month for customs clearance and extra import duties I considered it a lesson re-learnt. Don’t buy from outside the EU. As for Henry’s… I’ll say nothing.
A year and a half ago I found another one – clearly “spares or repair” – on Ebay. Both front and rear elements were in perfectly good condition.
To carry out the transplant operation I bought a specialist spanner wrench (not needed – d’oh) and a set rubber lens tools. The latter nameless rubber tools are funny looking solid translucent yellow rubber cylindrical thingamabobs – Google them. You’ll only need the 43-46mm one of those to help unscrew the front element.
To swap the rear element I found these instructions to repair a Nikkor 20mm 2.8 AFD very helpful. Note however that the front element is held in differently on the older non-D lens: three screws holding the outer front plastic ring on are under the rubber focusing ring which can be easy lifted off.
Yesterday I finally managed to finish the cannibalisation of one to t’other.