Hey @jo21! You can embed ConvertKit forms on Wix; we have a guide on that here30. The main limitation is that Wix embeds HTML content in iframes, which means that redirects (such as to a thank you page) don't work properly because they'll load within the iframe instead of in a new page/tab. The iframe limitation also means that our modal (popup) & slide-in forms won't work properly on Wix. As a result, when using Wix you should embed inline forms only, with a success message upon submit (i.e. not a redirect). There's some more context on this in the answers to this question about Wix21.
As for selling products — are you referring to selling ConvertKit Commerce products via a Wix website? If so, there shouldn't be an issue with the sales page being on Wix, but ultimately the checkout process would be via the ConvertKit product page for ConvertKit Commerce products. But you should be able link to your product page (the URL for which you can get from the Publish menu in the product builder) in a new tab so that your customers can check out from there. Let me know if I may have misunderstood anything about what you meant, though ☺️