Questions of how items compare with other items is best left to the main forums. That's why they are there. The for sale forums are basically just a trading post for business. Clarification on specs, like you did with the impedance question is fine.
Here's a long example of the many different things you might ask in a Head-fi transaction:
Seller says X Headphone for sale, includes X cable upgrade and manual, $400.
I've never heard of this seller so I look to see if he has a feedback thread. He does and it is satisfactory.
I think to myself, X headphone comes in 3 different impedances, so I ask in the thread what impedance it is and since it will benefit others besides me it is pertinent to have publicly listed.
Then I see the seller has Y headphones in his signature and I've been considering those also. It's not necessary for the public to have a long drawn out discourse on the virtues of the 2 headphones in question so I send the seller a PM.
Later I notice this headphone is supposed to come with a nice wooden case, but it isn't mentioned so I ask in the thread if the case comes with it because others would want to know also.
I'm still interested in these headphones and he's bumped the thread a few times while I've been deciding. I think $350 is a fair price for the item, but that only applies to me so I send the seller a PM offering him that price.
Seller won't budge on his price so I keep thinking about it and would like to see pictures so I can verify the condition to see if they are really worth $400 so I ask in the thread for pictures because others would want to see these also.
Seller posts the pictures and I notice a big ding on the side, which the seller has pointed out. I decide $400 is too much for a lightly damaged X headphone, but since it's my opinion I send the seller a pm reiterating my $350 offer stating the condition of the headphones.
Seller stands his ground on price so I decide to look elsewhere, perhaps Y headphones from someone else, and since it's my opinion they aren't worth $400 I keep it to myself and don't thread crap his thread saying how they aren't worth $400. The thread continues with others having a hassle free read in the thread knowing who might have been interested, but also that the seller has posted they are still available.
OR
Seller says he'll agree to $350. I post in the thread 'I'll buy them. PM sent for payment details" or something to that effect. Seller posts sold to me. End of thread.
The for sale forums aren't for chatting it up. It's meant to be a quick, efficient, system to sell and buy.