Are you getting the receiver too? If you don't need it maybe you can make an offer just for the speakers. Remember too, $150 is more than fair but zieglj01 did say they're more of a lateral move than an upgrade compared to what you have now. When you see Infinity keep an eye out for IL40s. Those are really good speakers and I've seen them on CL a few times for good prices. In fact all of the IL speakers are pretty good (I got a pair of IL30s for 10 bucks!), but the 50 and 60 both have powered sub sections and that always makes me a little nervous. The amp goes in one of those and you'll have fun looking for a replacement.
I like the 40s because they don't have the powered section, but they're still a 3 way speaker. I keep an active eye out for them on CL. My 30s are only 2 way, basically a giant bookshelf, but still sound really nice. I was pleasantly surprised when I got 'em home. I even went as far as to create a MultEQ file in the app just yesterday because I still pull them out and listen every once in a while.