Red flags for me is:
Bad covers of unknown authors. If it was a previously published book I go find the original cover and make my decision from that but I will pass over a book with bad cover that has never been published by a mainstream publisher.
Novellas under 60 pages. I just can't read something so short unless it's part of a series I am currently reading.
A book that has all 5 star reviews saying how amazing the book is and no real info about the actually book in the reviews or about anything they didn't like. I grow suspicious and think they are fake.
Covers with more then one hero on the cover. Some have like 10 guys and one girl!
This is a big red flag for me seeing I read mainly m/f romance. I tried the more spicier stuff but it's just not for me.
Paranormal fiction about heaven/hell with Angels/Demons taken from a biblical backstory. I just can't wrap my head around it. I know it's fiction but it's just the way I was raised. I had to give up a few series when they started going this way.
Anything that doesn't have a happily ever after ending. Real life is too depressing to read some sad story. I can't get that stuff by reading the news.