that means $5 part is sold for $10. It's exactly how that kind of thing works

That's actually a 50% profit margin. Also known as a 50% markup, even though you marked the price up by 100%.

	▲	antonvs 3 hours ago \| parent [-]
		It's a 50% profit margin because the profit ($5) is 50% of the selling price. It's a 100% markup because the selling price is 100% more than the cost. Some people might call it a 50% markup, but that's purely a deceptive tactic, or just ignorance.