Hmm. Not having the install discs is no biggy until it comes to needing to reformat a drive, or run diagnostics, when you will need them to get access to all of Disk Utilities features.
Now, it's quite possible that you won't ever need them, but if you don't have them, chances are you will, and buying them is not cheap (depending on the OS version).
Yes, eBay does have lower spec machines for more money, but those rarely sell, they are always on there, because to be honest, it's not too much of a further stretch to buy a Powerbook at those prices (I paid £300)
I think if he's willing to send the install discs over at a later date then you should go for it. £150 may be a little low though, but as he's a friend, mates rates apply