What is a cheapest and simplest way to set up phone extensions in an office (PBX)?
I do not know much about the PBX. Do you have a simple question. What would be the best way of providing a lifeline with a 3-4 home and extensions for sellers in my office. What used equipment I need to craigslist or eBay?
What our clients are saying?

You could consider a Talkswitch PBX system:
http://www.talkswitch.com/us/en/products/phoneSystems/280vs.aspx
cheap will cost you!
buy a new Panasonic electronic KSU from hello direct
1 line – PBX is a huge waste! a small KSU (5 line 8 station) is your better move. You can add automated attendant, music on hold etc.
another option is Asterick, but it works on a Linux OS and is free. The line cards are not too expensive if you find obsolite cards.
I agree that a 1 line KSU is a waste. You can probably find a small used KSU complete on E-bay. Panasonic is fine. The Avaya partner line works and is simple as they come (not all that simple btw) The docs are available online. That being said buy a small new system from a reputable dealer and it will save you in the long term. You can’t do business without phones.