The possible lack of profit that is left behind.

The barrier to entry is lower so they seem attractive, and to the person who knows what they are doing, they can be very profitable.

Determine local/state laws regarding wholesaling and determine if (1) you need to be licensed in the jurisdiction you’re considering and (2) if you’ll be able to operate profitably while remaining legally compliant2.

You'd have to be a builder to really reap any sort of profits from an area as such. 

A low price is no guarantee of profits. 

Just like any other investment, if you take into account all your costs and buy at the right price, you can still make a decent profit.

Keep in mind that landlords in expensive markets aren't making an profit (cash-flow) on rent.

This is maintained by the specific attorney’s office for the specific foreclosures they are performing.Although very profitable, Foreclosures are a poor choice for wholesaling, unless you have substantial capital.

For tax purposes, the profit is calculated as the sale price minus the cost basis (purchase price + renovations + transaction costs).

You're going to need 15-20 duplexes that are considered good deals to get to that kind of profit.