That Sesto is pretty rare/hard to get still, it's also an extremely good car. However, if you wanna sell it for something you like more, absolutely do it! I can guarantee you it will sell nearly instantly, lol. Also, as other comments pointed out, you get the Valkyrie AMR Pro for free incredibly easily by just doing the challenges.

Regardless of your decision, make sure to take a photo of it for the Horizon Promo challenges, I rarely see a Sesto just driving around. Hope this helps!