When I started using Twitter I used to tweet lots of my auctions as they were closing. One day I looked through my tweets and was horrified at the amount of plugs I was throwing out there. I also had to stop retweeting other members in bulk, far better picking one of their tweets at a time so you don't end up spamming peoples timelines.
The best way to tweet is to generalize ebid information so that it creates curiosity rather than showing individual listings. You can link to your shops and add an interesting note.
Examples: Did you know that you can list on this auction site for free #ebid.net
Sick of paying fees when selling online? Tired of ebay? Try
www.ebid.net and list for free #ebid
Like movies? Try
http://phoenix-dvd.ebid.net/ You might find one you are missing #ebid
Of course there are plenty of derivatives of the above and other types of tweets can be used at your own discretion.
Try using services like bitly.com to track your links and see if you are reaching people.
Remember that most peoples timelines move so fast that you need to grab their attention.