Need advice

Not that I'm trying to be cheap but I'm getting married this summer and am in the middle of shopping for Christmas. My fiancés sister lives Victoria Secrets but they are so expensive even I won't buy from them unless it's a major deal. There's this website called Mercari that people sell their used items. Most of the items are brand new or in excellent condition. And they have very reasonable prices. Is it ok to get her a gift from here or is it inappropriate?