Offer Price Etiquette

[John Matthews]

Is there a point where it's downright insulting if your offer is too low?

Let's say I'm considering a property with an asking price of $90,000. If, in order for the numbers to work out for me I determine I need to buy it for no more than $75,000. Let's also assume that the tax assessor has assessed it at $150,000. Is there any issue with offering $70,000 (assuming I'll get negotiated up to $75,000)? How about $60,000?

Secondly, can anyone point me to some resources on becoming a better negotiator?