To expand on what @Jon Holdman said, and given that you mentioned getting calls from people in foreclosure (or preforeclosure - same thing), I'd add the following points:
1. Depending on your location we are getting back to a time where not all preforeclosures are underwater and require a short sale. For a while there, almost all were. But pre-2008, most foreclosures were due to the 5 D's - Death, Disease, Divorce, Drugs, and Denial - where people end up in a situation where they are losing their home, rather than just being underwater and walking away. In these cases, where there is equity, it is likely possible to do deals.
2. One of the reasons that some advise to steer clear of these deals is that many states have equity protection laws to protect homeowners in foreclosure. For example Civil Code 1695 in California. It doesn't make it impossible to buy a preforeclosure, but it definitely adds some hoops that you need to jump through, and increases your risk should the homeowner have a change of heart. Even long after the purchase potentially.
3. Given 2 above, if you do these deals, I'd strongly advise being very upfront with the owner about your plans, whether to rent, fix and flip or wholesale. You'd b surprised by how many people are still willing to sell even when you fully disclose that you intend to make money. This is definitely not the place to be trying to convince them that it is worth less than it is (I personally don't think there is ever the place for that actually, but the risks are higher here).