With some carriers (Verizon, US Cellular, Sprint, to name a few), you may want to call in the ESN number for the phone and make sure there is no bill attached to it. Sellers often lack the time to do this themselves and will not refund on a phone that cannot be activated for this reason. This is one of the top reasons for bad transactions on
eBay. Try looking in the ad for the ESN first. If you don't see it, ask the seller. If you cannot get an ESN from a seller before buying a phone, I highly advise looking elsewhere.
If you are a US Cellular customer looking for a used phone, be sure to ask if the phone you are looking at has been activated on US Cellular service before. They are very proprietary and require the phone’s ESN to have been entered into their system before they will activate.
If you are a Cingular or AT&T customer, note that the two carriers are not automatically interchangeable. Both run primarily on GSM networks (Meaning they require SIM cards) and phones may be locked to accept either one SIM card or the other. If buying a phone that names the other carrier in the ad, make very sure it also says unlocked. Unlocked phones are those that are not carrier specific.
If you are a Verizon customer, you should be aware of the recent changes in their policies. Essentially they now tie cell phone ESN to the past user’s account if they leave an unpaid bill, meaning you cannot activate the phone if money was owed on the former account. Your best bet is to call in the ESN before buying.