Between eBay and Etsy, Salvation Army and Goodwill, local consignment stores (those around me even have different themes and categories staked out), swapping parties, Craigslist and the good old-fashioned act of picking stuff up off the curb, there is almost no reason to buy new these days.
It seems like every day there are new ways to more easily find great use stuff, but that doesn't mean everything should be bought used. Here are a few things that I won't buy used — and why — as well as the many items that I will pick up "pre-loved."
What not to buy used:
Blenders: It's really hard to tell how sharp a blender's cutting edges are until you get a unit home and try to puree a soup and end up with a gloppy mess. Unless it's a very high-quality device (like a VitaMix, which I would buy used because they are very expensive, super-high quality and replacement parts are easily available) it's worth it to buy a new blender. Blender motors also seem to slowly lose power, and it can be hard to tell over time just how weak they've gotten.
Pillows: Not only is is hard to tell how old a pillow might be, you have no idea what conditions it was kept in, and pillows can be havens for mold, bedbugs and bacteria (and animal and human dander if a cat or dog has used the pillow, for example). Since you have your face (and hence your nose and mouth) in a pillow, it's just not worth the health risk; even if you wash it, molds and other problems can stick around. If you find a gorgeous decorative pillow you really love, simply clean the exterior cover and then replace the interior pillow part.
Nonstick pots and pans: I don't use nonstick pots or pans due to the health issues that have been implicated in the coatings on them; Perfluorooctanoic acid or PFOA, the chemical used in the pans, can leach into food (and is found in more than 90 percent of Americans' bodies). The fumes from these pans can also kill pet birds in the vicinity. But even worse than new are Teflon or other non-stick pans that have even the tiniest scratches in their surfaces (as is often the case in used items). Those carcinogenic chemicals are leaching into your food once the surface of the pan has been scratched. Here is a good guide.
Underwear: I don't think this needs much explanation beyond the obvious gross factor, but bras especially get bent out of shape and stretched very easily, so spending the cash for a new one is worth it. That being said, I do buy camisoles and slips used (I love vintage lace). But I never buy socks, underwear, bras, stockings, tights or shapewear (Spanx, etc.).
What to absolutely buy used:
Obviously, there are many additional things that can be bought used than what I've listed here, but these are the categories in which I rarely — or never — buy new.
Clothing, accessories and even shoes (be sure to try them on first) are all completely fine to buy used. Be a bit careful with shoes as they can be "worn-in" to someone else's feet, but most of the used shoes that I've come across have only been worn a couple of times — or not at all. Follow my guidelines for thrifting here to get started.
Jewelry is like cars: the new stuff loses value as soon as you walk out of the store. Not to mention the seriously deleterious environmental impact that comes from mining precious metals and gems from the Earth. Vintage jewelry from every decade is widely available, and more unique than what you will find in a department store. You literally get more bang for your buck with used jewelry — a larger gem or more intricate details — for the same price you would pay for something of lesser quality that's new.
Bikes: A good visual inspection will give you most of the information you need about a bike, and used ones are so much cheaper than new that I can't ever imagine buying a new one, even if I could afford it. Keep an eye out for worn tires, rust on the frame, and the quality of the brakes — and check out and test the gears (but all of theses parts, save for the rust, are going to need replacement anyway, if you ride your bike on a regular basis). You also need to make sure the frame is in alignment, but other than that, it's hard to go wrong with a used bike.
Cars: I bought my 15-year-old Saab seven years ago on Ebay. (I live in Connecticut, where Saabs used to be pricy, even with high miles, so I found my car in Florida, where it was almost $2,000 cheaper than a comparable model close to home and it had no winter wear and tear.) It's one of the best cars I've ever owned. Be sure to do your research and spend the money for a CarFax report, but cars are another category of stuff I would never buy new — total waste of money.
Plates, flatware and glasses: I love to mix and match vintage glassware and set a table with pieces from the '20s, '30s, '40s, '50s, '60s, '70s, along with modern stuff. Choose a color scheme (all blues and whites) or a theme (florals, colored glass, etc.) to bring cohesiveness to a disparate set of dishes. Mixing and matching dishes or flatware is so much more interesting than matching sets, not to mention much less expensive. And if you break one, who cares? All dishes and glasses need is a good run through an extra-hot dishwasher.