But you'll need to something like the one below for the center channel connection. But If you know how to make this kind of connection then you can just get one 3.5 mm male jack and you the right channel then split the signal on a 3.5 mm female jack. The purpose of this cable is to split then combine the center channel to work with the suggest center speakers.
You think ebay is a dangerous place to buy? how old are you? I have seen a pair of PSB book shelf speakers for around 65 dollars used in the past. I'm not talking about the tower speakers, the bookshelf speakers are relatively cheap IF YOU LOOK HARD ENOUGH.
I'm saying Ebay is very safe if you know what your doing and who to buy from. Occasionally you can find great deals, sometimes no. Right now there are no good deals for PSB's or similar quality book shelf's to piece together a surround system. Craigslist sometime is decent for used stuff as well, and you can usually demo the stuff before buying. You have to put the time and effort into it, good deals aren't going to fall on your lap, but they are out there. I bought a par of B&W nautilus 803's for 1,500.00 on craigslist about a half year ago. If you know anything about speakers you'll know that's a great deal on great speakers.
So I have seen the speakers i speak of for the price I'm saying. Might take some digging into and a little driving, but possible. Or buy a pre built generic 5.1 system.
Congratulation with your good purchased, those are good speaker. I don't have problem with ebay or Craigslist or any other online transaction for that matter. Nevertheless, I usually not recommend used equipment unless good opportunity presents itself.