Malleries is a storefront for numerous sellers. They guarantee authenticity, but the trustworthiness and the service lies at the level of the individual seller.
When you look at an item, see who the seller is. Do a search here on the individual seller, for example "jane finds", "mightykismet", "girls got style" , etc. There are a few items that Malleries sells for Malleries. In those cases, there won't be a seller listed.
In the search function here, select "post" instead of "thread". If you don't find any feedback, it may mean they are new to Malleries. you can see how long they have been selling with Malleries at the bottom of the listing.
If you are comfortable with the feedback here and wish to proceed, authenticate the item following the required format and pictures. Even trustworthy sellers can make a mistake on authenticity.
Good luck with your items