I have no experience with flood vehicles, but all of my trucks have been salvage titled vehicles. There are plenty of good flood damaged vehicles out there brought back to life with no issues, but I'm sure there are also plenty of nightmares.
Insurance companies will total vehicles for a lot less damage than you would think. There are some factors that are instantly counted as a total loss, regardless of how much the cost to repair is. Stuff like flood damage, frame damage, or engine fires are quick to be totaled out by insurance companies. My current truck is a 2011 F-150 Lariat. The book value is about $35,000, but it was totaled out by an insurance company over $1,700 worth of minor fire damage.
So there may or may not be any issues with the car you're looking at. If he has been driving it for a few years with no issues, I wouldn't completely dismiss it immediately. But if he just recently fixed it and it only has a few miles on it since the repairs were done, I'd avoid it. And try to find out who did the repairs and what exactly was replaced. If you buy it, pay no more than 50% of book value. That's about what a salvage titled vehicle is worth. Sometimes, up to 60-65% may be ok if you're buying from a reputable shop that did the repairs themselves. In this case, since you don't know the seller or who did the repairs, err on the side of caution.
ETA: The fact that he doesn't mention the branded title in the ad would make me extra skeptical. If he's not being upfront about the title, I wouldn't want to buy from him.