Looking a bit more closely and reading thru the Brticycle ad, it sounds like a cheapie engine trans rebuild was done. From the ad, it sounds as though the whole engine and trans was stripped down then put back together with old pistons, rings valves,guides, rockers, crank, cams,lifters, untouched crank etc. Also no mention of new parts in clutch or primary drive.
The only new parts mentioned are seals and bushings plus mainbearings. It may be that all was in good condition or it may be that the owner avoided replacing the more costly items and went for a rebuild-lite, focusing on cosmetics.
There is no mention of the number of original miles, only that 570 miles have been covered since the complete rebuild, which sounds like more of a take apart for a look see then reassemble with minimum$ outlay rather than a rebuild.
If it happened to be a low miles original bike, most would prefer an untouched (non combat) factory assembled engine/trans that had not been pulled apart then reassembled, particularly if it is an amateur backyard type reassembly.
Glen