What are you, like, 12 or something ?
Oh, yeah, make sure you lay out your "terms" to them and by the love of God, point out that you WANT the "fair price". : ))
It is a little weird to start trying to buy something from someone who is replying to someone elses wtb thread.
But nobody has much money these days, if I had an a600 I wouldn't respond to this reverse auction. I'd just stick it on ebay at a reasonable starting price and see what happens.
I haven't re-read the rules to check, but I thought WTB threads were supposed to have a price on them.