i'm not a huge fan of either system as a buyer.
old classifieds: i loved how you could break out the guns by subcategory. if i wanted to look at semi-auto only, that's all i would see. aside from the fact that there was sometimes 4 pages of "featured" listings, it was easy to tell what was newly listed and what had been on there for a while. i didn't like the fact that the featured listings stayed featured long after the item was sold, or the fact that it was its own site.
new classifieds: all listings are grouped by major category (handgun/rifle, etc), can't break out into subcategories. listings can be bumped, so when anyone replies to it the listing goes to the top... it's tough to keep track of what's new and what keeps coming back to the top. yes i know that it says the date the ad was posted, but it's not as easy as it used to be. if i'm a buyer and i've already seen everything that was posted yesterday and decided that i don't want to buy any of it, there's no need for me to see the listings anymore. i want to see what's new. to see old ads that i'm not interested in keep bumping to the top is annoying. i do like where the link is located on the main site.
as a seller i can see it as advantageous. if no one bites on your ad you bump it to the top so it's fresh and always in view.
i've never sold anything but i have picked up a few things, so i'm biased. i typically don't have a lot of time to browse the classifieds. with the old system i could quickly check and see if anything new was posted. new system takes more time, so i find myself on there less often.
if i had to design the classifieds i'd have it filter/sortable, like most auction sites, and have it accessible the same way the current classifieds is now.
i could sort by handgun > all > most recent post first
or
i could sort by handgun > revolver only > newest listing first
etc...