eBay can offer some very attractive pricing on used phone systems but unless you really know what you are buying they can end up more expensive than if you consulted a specialist to begin with. There are a few things to look out for.
Brands - stick with the common brands such as Avaya, Nortel, Panasonic, Samsung or NEC. Other less common brands, while they may be very good systems, don't have as many installers so there may only be 1 or 2 installers in town who can charge virtually what they like and maybe nobody at all in remote areas.
History - these are used systems with an unknown history. What sort of environment have they been in such as temperature, humidity and dust. What sort of power surges have they been subjected to? Phone systems should be given at least as good an environment as a PC but in some cases we find them stuck out in a warehouse covered in dust and cobwebs. Our used phone systems have a known history. They were either sold new and serviced by us originally or they are New Old Stock systems from various sources that are a few years old but have never been installed before.
Configuration - digital or analogue lines, digital or IP extensions, voicemail and other features may or may nor be present and may be expensive to implement after the purchase. Some phone systems are fixed configuration with respect to lines and cannot be changed for another type.
Programming password - many phone systems come with the default password however if a specific password has been used it may be impossible to recover easily. For instance, on a BCM200 or 400 if the password is not known or cannot be guessed such as if it was an ex-Commander system then the hard drive needs to be re-imaged.
Model and firmware - there are many different models of Avaya IP Office for example. If you were looking at an old IP500 vs the current IP500v2 then be aware that a 2010 model IP500 cannot be upgraded to the latest firmware whereas a 2010 model IP500v2 can be upgraded to the current version and is then functionally equivalent to a brand new system. The IP500 may be worth purchasing if it is fully licensed but you would need a professional to be able to identify what licensing is in the system and if any phones are included they would be discontinued models.
In summary, think very carefully before spending your hard earned money on a questionable system because it looks like the bargain of the century. We have a number of refurbished phone systems in stock that can be configured to suit your requirements.