Says who? You? Sellers do not, in any way, deserve this. If you need to cancel an order, cancel the order. But please stop demanding that sellers have the ability to pick and choose which orders they complete. If a buyer places an order, they do so because they want to work with you. Denying them that chance is just rude, selfish, and inappropriate.
Yes, some buyers might “accidentally” place an order. If this happens, work with the buyer to correct the issue – maybe even convert that mistake into an order that you can deliver. And if that doesn’t work, you can easily cancel the order. Mistakes happen. That doesn’t mean sellers still deserve 100% delivery ratings just because they want them.
Take the bad with the good. If you deliver enough orders, it won’t matter much if you have to cancel a mistake here and there. If you’re not earning enough orders to offset a cancellation, then, maybe you have to work harder to bring in more customers.