Hi and welcome!
I've not done a huge amount of horse shopping in my time but based on my limited knowledge...
1. No - it is certainly not unreasonable to find a horse with no buck or no rear!
2. Giveaways - Only one of the 5 horses I've had in my lifetime ever bucked and he did it once after I had owned him a few years. I believe it was pain related but at the time, didn't consider that (10+ years ago) and he never did it again. So I don't know what specifically to watch out for...but can give you a few tips that worked for me.
A.) Go in with your head and not your heart. Uber hard to do, I know. But really think about the qualities you are looking for and don't compromise. Ever. And don't rush in to anything. Ever. Regardless of how hard a sell the owner might push on you!
B.) If possible, take a professional or knowledgeable friend with you for an unbiased opinion.
C.) Try the horse out more than once. Don't just go for a single "test drive". I'm not saying you should "spam" the seller with multiple requests to ride but I think that a serious buyer would be allowed to come back and ride more than once.
D.) A warning sign could be that when you arrive to see the horse, it is all tacked up and waiting for you. I always prefered to tack up the horse myself, after handling him/her on the ground, to get a sense of the horses general disposition.
Aaaannnnd, that is about all I can think of. I'm sure more knowledgeable members here will give you all sorts of great advice. Good luck in your search!