Secondhand shopping isn't something to be mulled over. There are great finds to be had, but if you wait too long, they'll often be found by someone else. (Vintage resale shops have made the secondhand market a pretty competitive place.) My advice is to buy now, think later. If the price is right, grab it and don't look back. If it's something you love, you'll find a way to make it work in your home.
If you're looking for a specific item (an Oriental rug, for example), I recommend hitting the refresh button as much as possible. The good stuff goes fast so stay on top of new listings by checking often and/or setting up notifications so that you'll be e-mailed when a new item is listed. Craigslist and eBay both offer this capability.
It's amazing the number of different descriptions that different people will come up with for the same item. My advice is to keep your search terms pretty broad. For example, I would search by "sofa" (although there may be many listings to sift through) instead of "green velvet sofa." I've seen many upholstered sofas described as fabric or cloth on Craigslist, for example, and would have never seen the listings if I had been more specific.
When I was interviewed for our BHG article about our home at the time, I was asked to describe my personal style. I answered "eclectic meets affordability." If something catches my eye, I determine if it has potential (or how unique it is) and if it's within my budget. As long as those things are there, I usually don't second-guess my purchase. I normally don't set out looking for a specific piece or something for a particular spot in my home. Instead, I keep an open mind and think about whether or not the asking price is worth the risk, in case it doesn't work in my home or I decide to resell it at some point.
It's easy to forget in a land of blogs and Pinterest that not every seller may realize the importance of clear, well-lit photographs. I've found some pretty amazing things that were represented online by a dark, fuzzy photo. Sometimes, these listings will get overlooked by other buyers who are quickly scanning the site. And, chances are that if the photo isn't professional, neither is the seller, which can often result in a really good deal.
This might sound pretty obvious, but I'm convinced you'll end up with a better deal if you mind your manners and treat the seller with respect. I can't tell you the number of times I've been at a garage sale or flea market and heard a buyer tell the seller everything that's wrong with the piece and then tell them the lowball price they're willing to pay. It's expected that most people will offer less than the asking price (I strongly recommend it!), but offering them a ridiculously low amount can start your negotiations off on the wrong foot.
If the price is way out of your budget, it never hurts to let the seller know what your best price is and leave your contact information, in case anything changes down the road.