A new Magento 2 Extension for My exiting webpage

i need a Magento 2 Extension for my exiting webpage it is working like [login to view URL] . there are a lot of samples pages. 1- Customer have one account but have 2 cart Selling and buying —First cart for to customers buying from our web page with default payment and check out (Default Magento 2 Cart System) —Second cart system for to customers want to sell their old products A-) User search yours old products Choose it And select some attribute like — Cosmetic Condition Too used Like new Broken — storage 8gb 16 gb .. .. .. B-)and system give a Offer to customers (For example System pay “like new” and storage “16 Gb” 100 Euro But for “ broken” System pay 50% less And “ 8gb” 20% less 100*0,5*0,2 = 10 Euro System pay only 10 Euro . ——In user selling cart We must know their information like email Bank account...for payment but We are not paying at this time. We will use default payment methods and a bonus system ———I mean with “bonus system “ We can pay with bonus to customer account for example 1 bonus is one 1,1 Euro if customer sell his old products and buy a new product from our wep page we don’t pay any money , admin put customers account the bonus . for me and for customers are better way...:) After check out , user sent the pack to our company. ——If system offer less then x Euro shipping is 5 Euro I will pay 5 Euro less at the payment . Or more then x Euro shipping is free . —— or fast checking processor (for example,Checking processor take 1 week but customer want fasted his money and customer receive y Euro less ) (I want to use a automatically generated shipping label. We will show it at the end of check out with their name and our company name )and any time later user can download pdf shipping label from his account. .....after the pack received to our company **** User receive a email and he can see same message on their account ——— if All things is correct we will pay the money with before selected payment methods and user receive a message from admin ———if not (for example user selected Cosmetic Condition “like new” but we checked , it is a used one ) admin sent a new offer less then first one customers receive a mail and he can see new offer and why is cheaper is ( admin can upload a few pictures to customer account or mail ... and user believes the admin ) ——-if user take this offer we can pay money or we can put bonus to his account . ——if user refuse this Offer we sent it to customers back . Thanks .