Is there a benchmark for the % off the original price for preloved goods? I.e. Should they at LEAST be 25% off the original price? Some are sold at same price as brand new

It's really up to you. There isn't any hard or fast rule for selling preloved goods. For me, if I've worn or used them at least once, I'd sell them off at 20% off, no matter how new. I've sold a brand new pair of shoes with tags and in box for 25% off just because I know it would sell off faster.

Think no benchmark. Well, it also depends on condition. It's basically depends on both the buyer and seller.

There’s no benchmark. It’s a free market so as long as there are takers at whichever price.

nope. up to the seller. depends on condition of the item.