That's a really good question. There are so many categories of items available for sale, it would be hard to find one site for everything. If you want a general idea of a price of an item, check a broad marketer like eBay for similar items that have recently sold. It won't be a perfect value for your item, but it would put you in the ballpark.
For possibly a more accurate value of your vintage clothing try Sammy Davis Vintage. For a fee, they will provide an appraisal of your specific piece. They can also provide links to social media groups that could help price your item for free.