"This shopkeeper offers beautiful things."
Translation:Cette commerçante propose de belles choses.
Everything is either masculine or feminine in French, so you must memorize “une chose” is feminine so it requires the feminine form of the adjective “belle” and “choses” is plural so we add an ‘s’. “Beaux” is the masculine plural form.
The masculine shopkeeper would be “Ce commerçant”, but everything is either masculine or feminine in french and the word for thing: “chose” is feminine, so it must take “belle” and the feminine plural is “belles choses.” We just have to learn the gender of each word as we learn and you can look it up in a dictionary when you don’t remember. https://dictionary.reverso.net/french-english/Chose
“cet” is used with a masculine word that starts with a vowel sound, for example “cet homme”.
I was marked wrong for making the shopkeeper masculine. I agree with Timi96160 below.
You should type your entire answer here. I keep having to guess what you might have put and which exercise you had for this sentence. If you put “Ce commerçant offre de belles choses.” in the exercise asking you to translate from English to French, then you could report it as also correct. If you had the write what you hear exercise, then you must put the feminine form and use the verb they said, because it would sound differently. The ending t is only pronounced in the feminine form when the e is added. https://www.thoughtco.com/french-demonstrative-adjectives-1368790