As someone who buys and sells, here is my take:
As a buyer, I want to pay as little as possible for as much (cards and/or perceived value) as possible. That's the sign of an educated consumer (anyone remember Sym's tagline of "an educated consumer is our best customer"?). You offer 100 cards for sale and I say "I'll pay $3", well, what do you expect? I know that paying 3 cents a card is a decent price (for me, the buyer).
As a seller, I research and figure out what I want to get back for what I'm trying to sell. Unlike the local supermarket or warehouse store, prices in the hobby are not "fixed". (Kind of like buying a car, prices are negotiable.) Unless I can't go any lower on a price (which I will state in my message to the buyer), there is room for negotiations. Now, as a seller, I'm going to try to get as much as I can for what I'm selling. Again, it pays to be educated about what you're dealing with.
The balance is finding that price that both parties are going to be okay with. If you were hoping for $1 per card (100-card lot example still) but the buyer only offered $3, well, there's a lot of room in there, and the two sides probably won't meet happily. But if you as the seller start out with "I'm hoping to get about $1 per card in this 100-card lot", then the buyer knows where they stand from the very start. It won't eliminate low ball offers, but it will reduce them. It may increase people "walking away" without even offering, but hey, that saves everyone time - and as the saying goes, time is money.
So to all the sellers out there, I'm starting my offer at $1 per 1,000 cards. Any takers? :)
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).