Turbo,
In would suggest your own domain name for each. Here's why: If you've been reading
PHPbay Pro Tips - Make Money on Ebay with PHPbay Pro | EPNTips.com you'll remember that in one of my posts i talked about creating an entire site around one niche will help you outrank sites that just have pages or sections about a niche.
If your site auctions are on hotbuys.com/wedding hotbuys.com/cellphones etc.. than google will see your site as a store with various products. But if you build a site called "weddingdresses.com and have 20 pages all about wedding dresses, google will see you as an entire site about wedding dresses. This will help you rank better than other site with just a wedding dress section (obviously other factors will be taken into account)
To illustrate this, my site
Fender Blues Junior amps - The unofficial site of Fender Blues Jr amps ranks in top 3-4 for the term "fender blues junior"
I'm ahead of huge online music stores with pages about this product (Guitar Center, ZZsounds, Musicians friend, etc). I'm even ahead of a HUGE review site with a whole section of reviews on blues juniors (harmony central).
I'm pretty sure it's because i built any entire site around my keyword and have relevant links with good anchors pointing to my homepage.
I doubt i would have the same results if i did all the same stuff as a sub page of an amp site (BuyAnAmp.com/fenderbluesjunior)
The trick is to create an entire site based on your niche, not just a section. Otherwise google sees yor home URL as a general resource, not specific to a product. So hotbuys.com home page would not show up for "wedding". Additionally Hotbuys.com will have inbound links for all kinds of stuff, Vs. a niche site which gets links with all related anchor tags...
Hope that helps. Just my $0.02