Do not buy in neighborhoods with large numbers of houses already on the market. There is too much competition, which usually ends in a price war.
Think long and hard before buying a house that smells bad. It could be pet urine that has soaked into the woodwork and can be eradicated only by ripping out the wood. Damp musty smells could be from a type of older construction that used absorbent materials to clad the outer walls of a house. Over the years the materials soak up moisture from the air like a sponge, and develop a bad odor. You will never get that smell out, unless you remove the cladding.
I do not like to buy from proud homeowners who like to show off their do-it-yourself projects. Usually the work is poorly performed or not up to code, and I am going to end up spending far more money than I planned.
For flippers, it is usually a bad idea to buy houses in historic districts or in subdivisions with strong architectural review committees. The reasons are the same, other people control your fate, they can be totally unreasonable, and they can delay approval for many months.
One friend of mine bought such a house, despite many warnings. His project was delayed one year over disputes concerning things like replacing some of the support piers. The piers were under the house and not seen by the public. He wanted to use concrete block. The historic district board wanted 100-year-old antique brick. He offered to compromise by facing antique brick over concrete block. They wanted antique brick for all of the work. This was just one example of all the problems he suffered. Eventually he remodeled the house, but it was three times more expensive than planned and took over two years.