I need to accomplish a unique shopping cart goal

This has been a thorn in my side for a while. I'm not ruling out any platforms / CMS / shopping carts at this point since I cant seem to find a clear solution anywhere. I want at the very least to find one feasible solution.

I'm trying to find a way to create a buyback shopping cart for a buyback site so I can purchase books from students as part of my business. This is the reverse of platforms now with items sold to consumers, but instead I'll be buying. The only related software I found on the market was a plugin for Magento oriented towards cellphone buyback sites: Buyback service (iPhones, iPads, iPods) | free & premium Magento extensions

Its the same principle but the process is eccentrically focused on phones with static prices (I'll be integrating a buyback algorithm) so that $250 would be more wisely spent elsewhere to create a custom solution for my own buyback process with books.

Here are sites implementing buyback carts:

http://ift.tt/2hoRWKc

http://ift.tt/2d9DcNZ

Sell Textbooks Online - Sell Textbooks Back For Cash

http://ift.tt/2gyVXds

Cheap Used Textbooks Online! | Rent Textbooks | Sell Textbooks Canada BookMob

In summary, are there any available shopping cart options where they are "buyback" carts instead? What would be my best bet for finding a buyback shopping cart? Where should I go to find a solution? What are your personal thoughts / suggestions?

Update: I've been told that the time it takes to modify any market shopping cart would exceed the time needed to write an entire custom shopping cart from scratch. Not sure if this is solid advice. I've also been told that the carts in these sites all are 100% custom - although im not sure if this means its built from the ground up or uses another shopping cart as a base to build off of.

What "caliber" of a project would a custom shopping cart like the one shown in these sites be? Is this a colossal task or it relatively easy for a competent programmer? What would a custom shopping cart like these generally cost? Any referrals on where I should source programmers to create me one? Would it be naive to seek a developer on Freelancer?