I think it'd be fine to offer less. What's the worst that can happen -- they can say no.
At least around here, people selling those cages think they're worth their weight in gold. There's one ad up right now that says "bought for $500, asking $250." Really? Cause I could buy a new one for that.
I think the average price people pay for a used one (depending on how much you want to spend) is $100-150. I've personally only paid $100 for each of mine, though I was definitely burnt on the last deal, as it was taken apart when I saw it and was missing parts (which wasn't clear because it was taken apart), so by the time I replaced those parts, I could've bought the cage new. But for the most part, I think $100-150 is a reasonable price.
I wouldn't personally pay $150 unless it was like brand spankin new, because it's a used cage, if I'm going to pay like a high percentage of its original price, I'll just suck it up and buy it new. I'll only buy used ones at a deal, which to me would be $100-125. But that's just me.
I think $180 is a bit high, I would ask if they'd take less.