I don't think I'd be overly concerned by one cable tie. If the rest of the car, history, owners, condition etc is what you are looking for, I wouldn't worry. My own car has four cable ties on it, due to clips breaking. Indeed, a lot of garages will use them if they can't find the right clip.
If in doubt, get the car checked by someone that knows. Have a good look round under the bonnet to see if there is any evidence of damage/missing parts/paint etc indicating something more serious like a previous accident poor repair. Of course, you would be wise the get a Car Vertical or HPi check etc to check if anything on record.