You say you have an AG-500, but the eBay receiver you references is an AG-550. Which do you have?
An AG-550 will support a center speaker and sub, the AG-500 will not and will not do what you want.
Even then the AG-550 has no HDMI connectivity and will be of limited use. A lot of units now only have HDMI outputs.
You have made the classic mistake of choosing equipment to fit a space, instead of making a space for the equipment required. You have gone about this backwards, and will pay a big penalty, more so going forward.