As Steve has said above, they do seem to be quite reliable
Neither my friend nor I have heard these tales of "fragile" gearboxes
*
To quote his response to your query .....
"Fragile ?". "What a ****". "Gearbox is fine, clunky change from new, renowned for it !" "Built like a tractor box and changes just like one". Just the way they are".
So there you have it RD, two endorsements from experienced, satisfied riders
As stated previously, the sale is "reluctant"
Tell your mate to snap it up quick at that price, before someone else does
* I have a Ducati with a gearbox that's been to the dentist, but I've been assured that even that is a rarity