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You're by far the first from doing so but this is why Sitefinity 6.0 will include a customizable sort/filter option for products that includes even custom fields. It's already finished and ready for 6.0 which is due out within the next couple weeks. We would definitely suggest waiting and upgrading to 6.0 as it provides a load of new features related not only to Ecommerce but Sitefinity as a whole.
With that being said you don't have to create your own product list from scratch hardly ever. Our widgets are ASP.NET custom controls so you can create a class and inherit from the ProductList and then override the necessary methods/provide your own methods.
Something like this:
CustomProductList : ProductsView
//find custom sort control -> find listview -> set sort expression
I hope this helps but I would definitely wait and upgrade to 6.0!
All the best,
the Telerik team
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