There are many reasons why you should be wary of cheap marketing lists that promise hundreds of thousands, if not millions of records, for next to no cost. More often than not they appear to be to good to turn down, the reality is, as with most things like this, alarm bells should be ringing.
Always ask for a sample, but make sure it is a sample of a specific criteria you request, and be sure to understand what the actual data contains, the field types, the population of these, how the data was sourced, how it is cleansed, does it have any guarantees, is it legally compliant and why is it so cheap? Also ask yourself how you are going to use it and what you are going to use it for.
How will you split down a large file of data to target the people you need to contact, presuming it comes with a contact name, can you select by business type and which medium are you going to use? What happens if the data fails to deliver and your marketing message is not received.......food for thought.