Ability to buy from customers?

I know the title sounds strange, but this is for a site that sells merchandise and memorabilia, but also buys memorabilia from customers.

Is anybody aware of a mod that allows for this easily?

There's no mod for that I'm aware of.

If you want to formalize the process, you could create a "product" with no cost which has attributes for the customer to tell you about the item they want to sell. These could be text input, dropdown/radiobutton selection of item type, file (picture) upload, or whatever. You can then reply by e-mail and work out the details.

For THIS SITE we use an external form management system (Machform by Appnitro). The seller sends details via the form as SHOWN HERE.