What's good offer that's not offensive?

[Theresa Zihlman]

I am wanting to get started in real estate for rental purposes and have a potential house purchase available to me. Without all the extra details, it's assessed at $135,000 but the owner says to make an offer, that we have them where we want them and the sooner the deal, the lower they will be willing to go. It's been on the market for nearly a year, they've decreased asking price a few times. The current asking price is $150,000. What's a fair offer? So many factors, I know...