You are not supposed to wait for the log book even for a minute - at least not from the guy selling you the car! Once the signing is done he should hand over to you the log book and a filled and signed transfer form from KRA which you also sign and forward to KRA. You should be waiting
for KRA to post you the new log book.
NB: The only time you should wait for the log book from the seller is if the car is "new" - an import from wherever - and KRA has not issued them with one yet! But this fellow of yours has a photocopy - which means he has the original! Where is it? Doing what? Unless this guy is following up the transfer with KRA on your behalf, then something is very fishy here!
Never count on making a good sale. Have the purchase price be so attractive that even a mediocre sale gives good returns.