First, it isn't so much about how much traffic, as about how many clicks. Lot of names out there see high traff but just can't seem to get the click revenue.
So you'll want to find out from the seller the CTR (click through rate) to see how many of those viewers are actually clicking the links, and then you'll want to ask how much revenue that is bringing, since some names generate much larger per-click rates than other names. You can buy a name with large traffic but such poor sponsored link rates that they're like .03 cents per click, and you can find names that get huge per-click links into the dollars each. I know there are some very rare bidwords that pay into the tens of dollars per click; the highest I've ever found I think was 'dna testing', which as I recall was in the $8 - 12 region per click.
To make a long story short, ask more questions in the 'traffic and monetization' section here. But Ask the seller to provide proof of stats, and then you should be paying anywhere from a year to a few years worth of revenue for the name. That's more or less standard... as long as it's a non-TM name as you stated. If he's selling you a name based on traffic, he MUST provide you with proof of traffic earnings. If he won't, then probably stay away, no matter how good the uniques seem to be.
