Let's face it. We aren't all Art experts. I'm not an art expert either. But I like to think I know a good bit about it, and looking around on eBay, there's some really nice stuff out there, and then there's some really not.

Some backstory on where I got taught. My art teacher in High School is an absolutely amazing artist. He does paintings and pottery, both of which are some of the most impressive things i've ever seen. If you ever happen to see an Art Deco-inspired, intricately carved, vividly colored vase, pot, plate, or oddly shaped clay creation with slabs, "roundy-rounds", "frogeggs" and other clay bits protruding every which way, chances are you've found one of his works. How to be sure? Check the bottom. It'll say "Bisaillon" on it. Which brings me to the first lesson of buying pottery and clay art.

• Check the bottom (or foot, in true potter-speak). A good foot is either completely dry (called dry-footing, usually found in stoneware, raku and commercially produced pottery) or completely glazed. Both of these have their advantages and disadvantages. When buying, think about these carefully. Firstoff - a dry foot is raw clay. It is hard as a rock, and will easily mar any sort of fine wooden furniture you put it on, unless of course it has been sanded perfectly smooth, which is all but impossible to do, unless you're a multinational pottery company and/or use amazing clay. Which is why commercially produced pottery, even when dry-footed, is smooth all over. When a foot is glazed, it will not mar furniature, but it may have things called stilt-sharps, which are sharp points of glass left from where the glaze flowed slightly onto the stilt which held it off the kiln shelf in firing. These buggers can be INCREDIBLY hard to grind off, but most people put the effort into it. Beware of those who don't. Another disadvantage to a glazed foot is that it may chip or the glaze may crack, or if you set it on something hot, it may melt the glaze and stick.
• Another thing to look for on the bottom is a signature. All of my pieces have a signature on them.
• So, coming off the bottom to the inside of the pot. Say you've found a beautiful, handmade, gold luster glazed goblet. That would be absolutely amazing to have wine in at a wedding or something! Whoa. Hold it. One word: LEAD. Believe it or not, I haven't seen a single law that regulated the ingredients in a pottery glaze, unless it was used in a school. That means if Cobalt, Cadmium, Lead, Barium Sulfate, Copper Carbonate, or even Arsenic make a glaze have a specific color or shine, then it's probably going into the recipie. I've seen a glaze that had Cesium, yes, Cesium in it. If you don't know what that is, they use it in radiation treatments for cancer patients. Sure, it was probably the absolute brightest red I've ever seen, but I'll take my chances not irradiating myself. Bottom line: if you want a piece that you want to eat off of, MAKE SURE it's dinnerware safe. My pieces always have something written on the foot saying "Food/Drink Safe!" or "NO COMSUMPTION!" or something to that effect. Unless it's one of the Teaboxes, in which case it would be complete idiocy to not be dinnerware safe.
• Third point is, if you are thinking of having the piece hold water for whatever reason, drinking, a vase, a teapot, holy water, whatever; if you want it to hold water, it MUST have the inside glazed. If the inside is left unglazed, and you put water in it, it may soak into tiny, nearly invisible cracks in the clay, permeate the clay, do strange things to the glaze...the possiblilities are endless. Just make sure it's glazed, and you're fine.
  
• Next up: Crazing. Sounds like glazing, huh? That's because it's related. First, a brief science lesson. When clay is fired, its chemistry changes, and it becomes something called a Bisque. No, not the thick New England soup, the clay. Well, usually there is a very small temperature range where the Bisque is absolutely perfect. This is called Maturity. Most clays have a range of a few cones (high-temperature ceramic bits used in kilns, they bend at specific temperatures) where they are acceptably mature, higher quality clays have lower ranges. Earthenware clays have a higher tolerance, whereas stoneware has a lower tolerance, generally speaking. Anyhow, if a Bisque is either overfired or underfired, when it is glazed, and fired again, the Bisque changes chemically again, which affects the glaze as well. When it cools, a crazed piece will have a few or even thousands of tiny cracks running through it. This is because either the glaze or clay shrank more during cooling, and one of them didn't quite fit. Severe crazing often looks like the piece is old and crackling, but beware. If it is badly crazed, it may eventually chip and peel, revealing very very sharp bits of glass. This is not guaranteed, only if the crazing is very pronounced or all over. Minor crazing can appear in even professionally-made pieces. Simply look closely, and if it looks extensive, beware.
• Now for something simple: Size. Photography, especially with a white backdrop and closeups, can make something look a lot bigger than it is. Or smaller for that matter. Always always always ask for size specs. Some of my art teacher's pieces are three feet high, and he once had someone buy one and send it back saying "I thought it was about two feet shorter".
• When buying wheel-made pots: Thrown pots can be beautiful and elegant, or the ugliest things you've ever seen. I've made some of both, but the majority have been of the latter type. Throwing (That means made on the wheel, don't ask me why) is one of the most difficult arts to master. Most people think it's simply pulling and pushing with your hands. It's far more than that. If your lump of clay is slightly off-center, the entire piece might end up ruined. That's why it's vital to look at a thrown piece from EVERY angle before you buy it. A bulge caused by the pot not being quite centered can easily be hid by taking pictures at correct angles. If it looks like a specific part is being hidden, be wary. There may be something not-quite-right about that pot.
  
• Another simple one: Cracks. Is the piece cracked? This seems simple but it's easy to overlook. Cracks are at once easy to see and hard to see. A big one can be seen easily from a direct angle...but after a bit of glazing and turning it in the photograph, it can be made invisible. A small crack can be invisible until you're sitting at dinner one day and say "Wait...was that always there?" Look closely, inspect and re-inspect. That's all there is to it.
• Lastly: Colors. Not all cameras and computer monitors are created equal, and then there's the wonderfulness of Photoshop. Is that green really going to match the green of your favorite tablecloth? Is that blue the same as your couch? Is that a matt finish or a gloss one that's going to catch the sun? Is that lustre gold or silver? Iridescence (Peacock-feather colors, change depending on what angle you look at it at) sometimes doesn't show up on a camera because of the lenses and auto-redeye removal. Make sure there's a return policy, so your $50 urn doesn't end up as a yard sale deal.

That's all I can think of for now. Be vigilant, and happy hunting.