eBay is definitely a buyer's market unless you have some uncommon or high demand item. If you are trying to sell just plain kits that anyone could buy at the local hobby store or at any of the popular online hobby shops, you'll be lucky to get 50% retail for it.
But try looking for some of the rarer Bandai Star Trek kits, and they go for quite a bit more than the original retail price.
My son wants a rare World of Warcraft mount (like a special horse, bear, tiger, etc. your character can ride) for his online game. This is an item that just looks different from other mounts in the game. There are several rare ones in the game, they go from $120-$1,000 or more. And this is a virtual item, it doesn't exist in real life. You get a code that unlocks it from the game and allows you ride around it in the game. It does nothing special over the standard mounts that you acquire within the game, it just looks different.