Just to be clear, the OP is not looking for a patent agent to file a patent (at least not at the moment) but to ensure that their design does not infringe an existing patent.
The common term for what you are seeking is a "freedom to operate" opinion, and they can be very expensive. As pointed out, sorting through hundreds of possibly relevant patents to insure your product does not infringe their claims will take hours... and hours. And so far, I suspect, you are only talking about American patents. Care to know what has been claimed in Canada, Australia, India, Brazil or Europe? Do it all over again and again. Want to know about China and Japan? Better add some translation fees in there. Bill that time at a few hundred dollars an hour and you can see it add up.
Allow me to point out, however, that assuming you want to produce or sell your product in the United States, and that there is an issued American patent that claims your particular device... that does not prevent you from producing and selling your item. Rather it merely gives the patent holder the right to prevent you from infringing upon their patent.
They have to tell you that they believe you are infringing. At that point it is up to you to decide whether to cease producing/selling the product, negotiate a license fee for using their intellectual property, or to tell them "see you in court" where you can either attempt to have their patent thrown out, or demonstrate that you do not infringe. (And if you thought the freedom to operate opinion was expensive....)
So if you do a reasonably decent check of the online patent databases, and don't find anything directly related to what you are doing, then I'd say go ahead and start selling it. If you can't afford a freedom to operate opinion then I suspect you are talking start-up expenditures in the hundreds or thousands of dollars and therefore have relatively little to lose. Freedom to Operate opinions are generally sought where start up expenses greatly exceed the cost of the FTO opinion, and thus the legal fees are only a component of the overall bill.
Go for it!
Jason
P.S. As to the "mail it to yourself" suggestion, not only are there far, far better ways to protect an invention (such as filing a low cost provisional patent) but that in 2013 the USA will finally catch up to the rest of the world and change from a "first to invent" to a "first to file" system.
http://en.wikipedia.org/wiki/First_t...irst_to_invent