Luckily parked at Sedo, so assuming you meet all other qualifications, there is no minimum commission through Sedo and you'll end up paying the flat 10% out of final value to Sedo if you use them as the broker (And this wasn't an outside of Sedo offer/soon-to-be transaction).
How to proceed depends on whether or not you'd be happy with $150. If you think it's worth more, counter with a highball, see where it goes. Some of my largest sales have been when I didn't value the domain as much as the buyer, but I threw a highball back and got a bite at a higher amount than my own "personal-reserve".
However, if you're thrilled with $150, you can simply accept the offer and start the $/DN swaperoo.
-Allan