Dear SRazbadouski,
I agree with Tim's suggestion of using eBay or a similar online auction service that's already set up for this. Does that fit with your thinking about a "plug-in"?
Also, you need to think about whether the online silent auction items are being sold just online, just offline, or both. I looked into this for a charity event I was helping with and we concluded that allowing people to bid on- and offline for the same items wasn't practical. It could be done if the live auction bidders had some way of bidding online (e.g., mobile phone apps), but we couldn't require that as a prerequisite. Otherwise, someone would need to be constantly keeping the online and offline bid "sheets" in sync. Since most action at any auction takes place within the last two minutes, and much of that within the last 30 seconds, this isn't practical. The best compromise seemed to be having some items only online and others only offline. But the offline items could be promoted online to drum up interest.
Robert