Unless you like paying nearly double what you need to pay for the exact same brand new cellphone, if you are buying a phone that's been out a year or so, like the Samsung Galaxy S5 (which is a fabulous, gorgeous phone with a brilliant display, lightening-fast processor, rapid-charging from empty to full in 1.5H, and removable battery with great battery life), instead of paying more than $500 to your carrier, you can get a brand-new in-the-box totally compatible unlocked Samsung Galaxy S5 for under $300 from any number of places. Here are some things to keep in mind:
1. Pick a seller that has a good no-questions-asked return policy, such as Best Buy (15 days) or Amazon (30 days).
2. Amazon seems to have better prices than Best Buy. eBay has deeper discounts than Amazon but it's not clear that any seller has a no-questions-asked return policy and some have restocking fees of 20% and perhaps more.
3. If buying through Amazon, no matter who the actual seller is, be sure the order is "fulfilled by Amazon."
4. Before ordering the phone you want, call your carrier with the exact model number to insure total compatibility. This will take just a minute or so and your carrier (AT&T at least) will be very nice and helpful.
5. You can also expect (at least from AT&T) excellent post-purchase support for your new phone. If the new phone takes the same size sim card as your current phone, you likely can set up your new phone at home with no support. If the new phone needs a different size sim car, just go to the carrier's store and they (at least AT&T) will quickly install the new sim car and set up your new phone at no cost (the sim card is free (at least with AT&T) and there is no activation fee (at least with AT&T).
6. The discounts on certified refurbished phones are even greater, but our Consumer Counseling Department recommends going the new-phone route (e.g., with a refurbished phone, you might have designations from some other carrier).
7. Be sure to include accessories (e.g., screen protectors, fast-charging cables, cases) when you place the order so you will get free shipping on them and they will be to you when your new phone arrives).
8. If you want to buy insurance for your phone, go to SquareTrade.com within 30 days of buying your new phone. It's hard to see where it's worth it though when you consider, among other things, the deductible.
[Note: While unlocked iPhones are available, it does not seem the discounts on them are such to make going the unlock route worth it]