I don't sell, but I do pay attention to what sellers in my area are doing.
First, I see alot of two-tiered pricing: Mixed hardwoods = $XXX/cord, All Red Oak = $XXX/cord. Mixed hardwoods usually mean there's some poplar, cherry, walnut, ash, and/or locust mixed in. If I were buying, I'd ask how much of each.
The rounds need to be split small enough to fit in the "average" stove or insert; so they usually come 16" length by 4-5", regardless of whether its a quarter, third, half, or whatever. Most guys with bigger stoves tend to harvest their own wood rather than pay for it.
When I do deliver wood (I give it away), I always mix in smaller diameter stuff. First, because I have it. Second, because it helps with getting the fires started. For that same reason, I usually always include some smaller split poplar.