Why would someone sell a positive cash flow real estate?

Can you answer xoxohthdotcom’s question about Factoring?:

I don’t understand why someone would sell a positive cash flow real estate. I found a whole bunch of properties. Some of them have tenants already inside.

What should I be weary about? What should I look for?
In addition:

It’s a nice area. A college town. There are a couple foreclosures in the area and a couple people are selling houses that have a positive cash flow. Something is going on in that city.

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