You can consider the following points while selecting the ready-made software:

Is it fully customizable?
Is it pre-integrated advanced APIs?
Is it user-friendly and scalable?
Reviews and the number of platforms powered by the software
Is it scalable & secure?
Is it Confidential?
Is it Feature Rich Solution
Free Technical Support
pre-integrated APIs?

You need to check all these points before selecting any ready-made eCommerce software.

Here I am sharing a software name .i.e. Yo!kart (https://www.yo-kart.com/) that has all these features. You can launch a robust online eCommerce marketplace. 100% custom & scalable multi-vendor eCommerce software.