If you put something up for sale in the classifieds, we expect you to be able to complete the sale if someone uses the 'buy now' button. So if you sell something in the classifieds, you should not also put it up for sale on ebay or another sales site.
If your item would sell on both sites, one buyer would have to be disapointed. And since ebay has pretty draconian measures in place to deal with such situations, the SRP buyers would get the short end of the stick.
