Check whether the product page links a brand-issued size chart rather than a generic store-wide one, and read a handful of recent reviews mentioning fit specifically — sizing consistency is set by the brand, not the platform selling it.
It varies by retailer and sometimes by item, and the terms are set on the product or checkout page rather than being uniform sitewide. Check before ordering multiple sizes with the intention of returning some.
Regional brands typically skip the import fees and cross-border shipping that inflate the price of the same category imported, but imported labels win on selection for styles regional brands do not carry at all.
The return policy first, since final-sale items are common during deep discounts and cannot be returned even in the original size. The discount percentage is the second thing to check, not the first.