I've sold lots but yes, 60% is unrealistic unless it's top-end stuff. Bugaboo, Stokke, Boori and Bloom stuff sells well. Cots, high chairs, nursery furniture, buggies. I've had 50% or a bit more for things like that.
Clothes go for peanuts unless they are good brands. And I don't mean Next, which so many people list as "designer" on eBay, I mean Boden, Joules and proper designer clothes. You might get a few quid per item then. Bundles are easier to sell.
What have you got to sell?
With postage, it might be worth looking to do a local nearly-new sale? Rather than NCT sales, I tried a couple of the sales organised by Mum2MumMarket and similar and had a decent amount of success.
I've also heard that car boot sales can be successful for stuff like clothes, but mostly you just get 50p per item for basic Mothercare clothes.