Take a look at this one for Liberty Reserve : http://wordpress.org/extend/plugins/liberty-reserve-payment-wpplugin/installation/ The installation instructions are similar for most all WP plugins. This page : http://wordpress.org/extend/plugins/paid-downloads/installation/ says: Have a good day.
I also heard a person saying me, USE OPENcart .. i downloaded that, and i intalled that on my local machine.. that's was amazing, that was supporting lots of payment gateways, also Payza..
Yes, OPENcart is a good one but I believe there are few more plugins available out there.. so just use the search function...