For anything that's a sure seller (Little Tykes blowmold playthings are a great example) go with half the retail price. Dealers can't make any money at that price level. Forget about ebay because these items are large and shipping via normal means kills the value. A good rule of thumb is to haggle down no more than 15% of your asking price on things that will sell, and target 10%. Counter, counter, counter. Remember that *you* hold the goods. The only exception I give is if someone wants several big ticket items. I'll sometimes let them all go at once for 20% less, but that's about it. There's always Sunday if need be. Again, it depends what your priority is, cleaning out the house or collecting cash. If you know it will sell, than hold out, especially first thing in the morning when the dealers converge. It is tempting to make the quick sale and start building the wad of cash. Dealers know this and try and get all the good stuff in one lot at a steal of a price. They will also find other interesting things on your property and bid for them even though they are not for sale. Beware, it's easy to fall into this trap. Hold out for your money. Rest assured that if they don't get the margin they need, they will move on to the next vic... - ahem - seller soon enough.
For everyday stuff, I would start with 25% of retail price and go down from there. These are things that you want to get rid of that not every buyer is going to check out. For larger bulky items that you would take to the dump after the sale, try and get a few bucks for it and take anything you can get. The less that goes to the dump the better.
Some might think that I'm being too harsh to dealers. Hey, I'm one of them! I'm just trying to give you an idea of what it is like from the other side of the fence, so that you may maximize your profits.
Best of luck,
Chris