Hi, I have rich experience in Reactjs and Java. I'd like to do this job. Here is the answer to your questions.
1) Do You intend to base your development on some existing tools (e.g. reverse auction scripts, e-commerce platforms or CMS)?
Answer: I have a demo commerce website, I will develop based it. You could view it via http://cloud.dobaor.com. Username/passwd:factory/factory.
2) How much will it cost per hour to order changes from You after the product has been delivered to You?
3) How much time You will need to develop the system?
I suppose to finish it in 1 month, you will paid me 350 EUR(includ 35 EUR for the freelancer)
4) What is your experience in developing earlier similar systems?
My demo website built with Reactjs and Spring Boot, delopyed in Docker container. Include category list, shopping cart, agency management, product management and plenty of other modules. I have really experience in developing website like this.