So, a short form of the 6 services:
My top choice, as the services are currently, semi tied with apez. Beats out meta-life because that excluded itself from being useful to me. Beats out apez (somewhat) because it has somewhat better bulk listing. Listing itself is weird and takes a lot of getting used to, may in fact be harder to list individual items than any other service, so it's not super awesome.
Semi ties with MVX, in fact I'm still waffling as to which of these two I prefer. MVX has easier and more interesting bulk listings (which I like...but also kinda don't like, because you can't SEE everything at once easily), but apez has some semi decent bulk listing features, so long as you never, ever, EVER change your mind. I would say it's easier set up than MVX for singles, LOVE the collections way of grouping items.
Mostly a pretty damn great service. Pretty damn near totally useless for me because of the image thing. Wish it would have website image uploads, and apez's collections- with a few very minor interface tweaks on top of that meta would quite possibly be my perfect service.
Of the four I could get up and running, slapt was the most lackluster. No bulk features, just about as clunky listing wise as xstreet (and if I have to choose something else, I'm choosing something with BETTER features), and interface problems that make it almost unusable for me because I have problems figuring out what things say at all.
Suffers from the "if I need to move, I should move to something that makes my life easier" problem- listing looks similarly clunky, about on par with xstreet. Geared towards the Japanese market, _and_ has a deal on commissions for low priced items, so, as a consumer, this is quite possibly a hidden gem full of holy crap that's amazing and I need it right now. It's just not something I'd use for selling on, currently.
Not yet open (the website itself only launched at all the beginning of last month), so out of the running.