I have a couple of inactive cell phones (Motorola 120e and Samsung
> GS-x426) that are lying around because I have taken phone
> upgrades. They are blanked out and ready for service, and I want to
> sell them on E-Bay. Howver, prospective buyers will want to know what
> cellular service providers the equipment will work with. How can I
> learn this? The Motorola was originally used in the Verizon Wireless
> network, and the Samsung was used in the AT&T Wireless network, but I
> have a feeling other providers could work with these phones. More
> network compatibility means a higher number of potential buyers.
As the French might say it depend. The Verizon CDMA phone will likely work only on another CDMA carrier that allows their customers to attach a non-native CDMA equipment to their network. Many networks such as Sprint PCS which is also a CDMA network will not allow connection of any device that they either do not sell or has been formerly on their network. They will check their database to see whether the ESN (electronic serial number) is there. If it is not many carriers will not activate a handset that has not been on their network.
In the case of the AT&T Wireless phone it will depend on which kind of phone it is. If it is a TDMA phone you will only be able to use it on AT&T Wireless' TDMA network and not on any other network such as cingular's TDMA network. Cingular/AT&T will not activate any new TDMA phone service though I imagine you can switch the ESN from what someone already has to another phone which has been on the AT&T Wireless network.
If it was on the AT&T Wireless GSM network you could use it on another GSM network provided that the SIM lock is defeated. AT&T Wireless did not and will not give unlock codes for the GSM phones that it sold and you either have to go to sites that will "remotely unlock" or find someone to flex/flash the phone so that you can use it on another network. Also with older GSM phones that AT&T Wireless sold they were single band 1900 only. To get the best possible service you really should get a dual band 850/1900 phone.
If you know which was the original provider you need to let your perspective buyer know this. Many buyers are well aware of what will and will not work for them so I'd strongly advise giving as much information as possible including which networks (and technology if you know it) the phone uses. Also you need to be sure and give the correct full model number since some models have the same model number but are for different technologies e.g. Motorola V60c, t or g depending on whether it's CDMA, TDMA or GSM.