Hello welcome to the forums. this bike might be a tad new for the C&V forum but, this forum is much more active and friendlier than most of the others. the '02 Brava looks like a nice bike. did the seller say why he changed the cranks?
as to the value that is very subjective. 400 would be good if the bike is a ' garage queen' and saw little use. if the bike was used alot, commuter, training (esp on a indoor trainer) and parts are wearing it it isn't a deal. have you seen the bike? can you get some good pics? the BB area, rear derailleur, and dropouts, plus the rims. that way we can get an idea of what kind of use it and care it had.
edit
$750? well the bike had better be pristine to asking 400 for it, it is 8+ years old. are there any accesories? real nice computer, pump, bottlecages?
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk