Personally I have no idea why it"s called a flea market or whether it has anything to do with fleas at all. What I do know however, is that you can find some good specimens of coins and at reasonable prices in some of these flea markets.
The only thing is that it requires some patience on your part as you might need to go through quite a few collections or sets before you find anything that"s of any worth to you. This can be beneficial though as you might come across a few coins which will indeed enhance your own collection.
And if worst comes to worst, and you come back with fleas instead of coins, you can at least pick up a couple of knick-knacks on the way, to further clutter up your home. You might even find an interesting piggy bank or two in which you can store some of your initial findings or run of the mill coins.