How do you decide which movies are worth buying?

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How do you decide which movies are worth keeping might be a better question. I sell on around a third of my purchases. I've gotten rid of loads of blu-rays over the years. I'm quite often surprised at just how many films some of the other members own. Makes me wonder if they automatically keep absolutely everything that they pick up. Personally I expect strong replay value from films in my collection. Only exception is Dario Argento's really weak last couple of films where I only keep them so I can have a complete Argento collection.