It depends on the tools as well as the seller.
The average for Lie-Nielsen planes on eBay over the past year is 87% of MSRP. And that's ignoring the fact that they typical eBay shipping charge is 2 or 3 times what LN charges. Of course, they are a highly coveted premium plane that has a lifetime warranty. The popular ones that come up frequently average between 80 and 85%. The reason Lie-Nielsen tools command such a high percentage of retail on eBay is because they never discount.
You can pretty much always get decent deal on most tools if you research what things actually sell for and exercise some patience. Most stores sell returns, even new and unopened, at a discount. "Factory refurbs" are often new and unused returns and can be had for good prices. Retailers (and manufacturers) have sales giving good prices from time to time. I just got a flyer from WoodCraft with 10 or 15% discounts on most items the weekend of 2/3 March. Some, like Delta for example, have periodic rebates. I got about 10% off on my lathe via rebates.
- Mike
Si vis pacem, para bellum