As with any site, there are going to be negative and positive aspects. I personally have had a good experience using Listia. I got a good amount of credits when I started my Listia account because they give you 500 credits and because I signed up through a Facebook friend's referral link which gave me bonus credits. I did some surveys online and got credits, and have listed items (small items that don't cost much to ship).
I try to only bid on items with no shipping or very little shipping. I have gotten rolls of duct tape for my daughter, Betsey Johnson jewelry, make-up, etc. I got all of these things for free. Luckily I work a desk job and can lurk my watchlist while I'm working and pounce on items last minute when there's only seconds to go and win things for a reasonable amount of credits.
I wouldn't list big items I want to get rid of as shipping for those things wouldn't be worth it for me and like other people have stated, most people don't bid on auctions that require payment for shipping. I list jewelry, single clothing items, etc.
The only thing that upset me and discouraged me from Listia was their feedback system. I ordered some Betsey Johnson items from someone who claimed it was authentic. Out of the whole lot of 6 items (that I got fairly cheap) I had a few items that I didn't particularly see as my style so I relisted them. I ended up getting bad feedback (both items purchased from the same person) because she said they were fake. Even though I assured her I did not know the items were fake (I really didn't, they looked legit to me) and I gave her back her credits and $2 shipping, she still ruined my reputation over it and I definitely didn't deserve it. I should have disputed it but I didn't realize all this would happen. I'm still working up to at least have enough items listed that I can get a 99% feedback rating. But I wonder how this woman posting fake Betsey (she has hundreds of items on two different accounts and I know other people have told me that they were fake after the fact) has a 100% feedback... makes no sense! Oh well...
If you're careful, Listia can be an awesome site. I'm a bit addicted myself :)