Receiving an offer is a good benchmark for a domain, sure. But at the end of the day, the only thing you can go on is real data, based on real market prices of similar domains. And unless your domains are liquid, you have no way of knowing what price you can get for them now, or in 10 to 100 years from now.
In my case... I have 29 names listed on Brandbucket, which I could realistically value between $10 to $40 each (because there is an established market for these). My other 100 names are a combination of random brandables or EMDs. Yes, I price them in the XXX to XXXX range in the aftermarket, but if I were to sell them in bulk today, would anyone even give me $10 per domain?? I have no guarantee of knowing, and until I do, those domains are more liability than asset. My combined asking price for them would be over $100K, but in reality they're worth next to nothing.