Most of the live chats use XMPP at the backend normally through a Jabber server, the front client is normally a javascript client, which communicates to the server via BOSH or web sockets.
While I don't know of any project that specifically set up as sales chat system, their are several open source jabber servers such as EJabberd and numerous BOSH clients out there, the paid services are obviously designed to give you much more control of the operators, multi user pickup etc but these can be found using added on extras.
Obviously going down this route you have to maintain the jabber server, so will need a suitable host, set up the additional plugins + setup the javascript client. Meaning a lot more work, though you can customise it far more.
The reason the paid services are there is even for tech companies it can be a lot of work, while for example the company I run is perfectly capable of managing such a stack and have at times pondered doing so the guys at Olark (who we use) make it so easy, and are so helpful and responsive it makes it easy to justify the expense. The live sales aspect paid for itself within hours of going live and continues to do so.