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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%.

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.

It's the industry-standard (for grocery stores at least) term; doesn't mean it isn't wrong, but it's the way they use it.

Could be deceptive, could be ignorance, but it's multiple unrelated grocery store owners, talking to me as a seller.