I have built a Modx Evolution eCommerce shop with FoxyCart before.
That worked really well in my situation (a food box delivery business).
The advantage of using Evo with FoxyCart is that you can use Evo's fantastic Snippets, Templating and TVs for anything you need (for example - a product's price and options are TVs within an Evo template - very easy when you know Evo). FoxyCart takes care of the cart (in a responsive overlay), the customer login, the transactions and payments on their server.
You don't quite have full control over order / customer management like you would in a standalone eCommerce system but FoxyCart has an API and no doubt some tricks where they might be able to pull the data back into the Evolution web users section (I didn't do that in my application at the time but they may be able to help with that??).
FoxyCart's support team are awesome and will be able to answer any questions you may have - they generously provided me with various code snippets for our requirements throughout the project that could be plugged straight into our Evo site.
Definitely worth a look if you love working with Evo.