I buy a great deal of jewelry from people and swear by the acid tests. As mentioned, having a magnet, a loupe, and the acids means you'll almost always have a firm idea on what has been presented for sale.
Any magnet, of decent strength, works. I happen to use one that is encased in rubber with a pull strength of 32 pounds. Here's the link to the one I use. The magnet will allow you to move through fake jewelry rather quickly. Only clasps or joints should show any signs of being magnetic. Ideally, they aren't magnetic either.
http://www.kjmagnetics.com/proddetail.asp?prod=DX08BR-N52
For a loupe, buy one of decent quality, it's worth the investment for the amount of time you'll spend looking through that lens. The cheap knockoffs are just that, cheap. I use a belomo 10x triplet. That's sufficient for what you'll be using it for. I think it was about $30-40 USD. The loupe is for more than just reading hallmarks, though the hallmark gives you a beginning for acid testing. Look closely at scratches on the jewelry, you might be able to determine a piece is only gold-filled, just from observation. Do not blindly trust the hallmark!
http://www.belomo.us/
The acid kit mentioned is perfectly fine. You can even learn to make your own acids. That information is on this forum, but it's just as feasible to buy the kit. With most kits you get 10k, 14k, 18k, and 22k. Usually, you also get acids to test silver and platinum. The box shown in the link is one I use to store my acids, it's not big enough for all of your testing supplies, but it's a start. I found it cheaper to buy the box, the scale, and the acids separately, but at least you have an idea of about what they should cost. Note that I think the stone in that set is crap, and the scale, while not entirely accurate, is serviceable. For someone in your position, where customers come to you, I'd employ a much more accurate scale.
http://www.ebay.com/itm/Precious-Metals-and-Diamond-Testing-Kit-WOODEN-BOX-for-Gold-Silver-Test-Acids-/131460328999?pt=LH_DefaultDomain_0&hash=item1e9ba56e27
I would also suggest a quality stone, either an Arkansas or a Blue Danube. Any testing stone is perfectly workable but one necessary detail is to buy a stone that can accommodate several concurrent tests. Here's an example of a size that would suit your needs. Be sure to prepare your stone before you begin using it.
http://www.starstruckllc.com/page/SSL/PROD/SST-743?gold_testers
The final item that I recommend is gold test needles. You'll need these to ensure that your acids are still working. They provide a known comparison so that you can be confident in your acid testing. Use them when testing jewelry and you can see if the acid attacks each line to the same effect. Typically, they should be about 30 dollars for three needles, 10k, 14k, and 18k. You can decide if you want five or three. I only use three, and that has been enough for the years I've been buying jewelry.
http://www.starstruckllc.com/page/SSL/PROD/SST-714
And finally, this link contains some reading that will help you. The gold testing.pdf is a great resource written by a pawnbroker.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=60&t=21259&p=219367&hilit=how+to+test+jewelry#p219367
