What are these cash only deals?

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I've come across a few multi-family properties that are listed as cash only. They list their terms as:

[i]Payment for the full purchase amount plus the closing costs are to be wired within 48 hours of an accepted offer.

You must do all due diligence and ask all questions before your offer is accepted. Your funds are to be in our possession within 48 hours of an accepted offer.[/i]

What kinds of sales are these? This is for a $600k property too. Should they be avoided?

Thanks!