its nice to think that maybe those that make a lot of sales from sites will pay to advertise their goods, but generally people don't.
we have the advertisers packages which are offered for commercial sellers so that they can post lists of what they have. like mikew, he breeds/resells a lot, so wanted to pay for an advertisers package, but it was hard to decide whether he needed to or not, where do you draw the line between those that breed a few fish (geeman, irene etc) and someone who would be classified as a commercial seller?, luckily for us, Mike said he wanted to pay for a package so we didn't have to think too hard about it.
ultimately, for the future of the hobby, we want to promote breeding on here, so i would see it as counter productive to charge breeders to sell on here.
we want everything to be accessable to all members, for free, but as every penny for the site had to come from mine and Lisa's pockets, we needed to introduce some fee's which is where the need for the advertisers package comes from, we dont want to have to charge for anything, but at the end of the day, the site isn't cheap to run, and with limited numbers of subscribing premium members, we decided that those set to make money from the site should be the ones that help fund it, and for everyone else, they have the option to pay, in the way of premium memberships, to help us fund the site.
saying that, i do think there could be a call for something like your idea, maybe without the fee. will post in the staff lounge and see how the idea sits for something like a tank bred/ breeders section in each of the classifieds.