My first reaction was childish. "Fill your bookmarks with grotesque porn links and merge the bookmarks!"
If I'm understanding right, Chrome asks you before it goes ahead and merges the bookmarks, so why not try logging in again and seeing if the message appears? If it doesn't I would assume that to mean that there is no merging happening between the two accounts.
I would probably just reinstall the whole thing anyway. Who knows what the person before you was using the computer for, or if they've gone and put some background software in place (although if he wasn't clever enough to remove his account details I would imagine he hasn't done anything intentionally malicious).
The person who sold it was so computer illiterate she sold it as "broken" for insanely cheap (£40). Literally all it needed was a system restore back to the first system restore and remove all the spyware that her kids installed which stopped the machine from booting (or, I assumed that was the reason it wasn't booting, anyway). She only got the laptop a few weeks ago from a guy who sold it on to her, and that guy seemed to have done a complete re-install before selling it on, so... yeah, I didn't see much of a need.
The only real thing stopping me from doing a re-format is that it's a serial OEM key and the key's faded off of the Microsoft license sticker. Unless anybody knows a way of getting the actual serial key from the computer itself reformatting would leave me without a copy of windows to reinstall (plus, it's an Ultimate version, which is a disc I don't have).