For Professional Use Only

This phrase is usually stamped on the back of professional electronics equipment, like monitors, VCR's, and such. It is rather stupid to stamp that there, since you can use such a monitor or VCR pretty much anywhere. If I want to buy one at the flea market and plug it into my VCR, I should damn well be able to.

That phrase is there to protect you from the product. The product may be dangerous in some way that a lay-person may not even conceive of.

For example, your flea market "Professional" monitor may be pumping out more (or stranger) radiation than is acceptable in the consumer version.