Need help dating vintage looking dress with no tags

We do not give advice on valuation - a selling price will depend on many things like the selling venue (online, brick and mortar shop etc. etc.), condition of the dress and others. You could for example search sold items on ebay or other online selling platforms to get an idea.
To help determing what kind of fibre the fabric of your dress is, you could try a burn test: https://vintagefashionguild.org/determining-fiber/