I would recommend a share link, that would give the possibility to customers who do not have an account to enter the seller's catalog and create their "shopping cart", a purchase order ( similar to the purchase order you already have but created by the client not the owner/seller) with the items that are added to "items" and that purchase order would be automaticcaly added to the sellers profile as a "request to approve" so the seller can create the invoice (such as in Brandboom and other digital catalog platforms "), thus improving, smoothening and streamlining the customer-seller / items-purchase / buy-sell.

A link to share basically to create an order.

if I wanted to buy three items from someone who has a zoho account, I would ask them for a link that would send me directly to their "items" and this would let me create an order wich then they would turn into an invoice. it's much like e-commerce but more like a digital catalog ordering page..

this would make easier my relation with my clients, they could make their orders without having to dictate them to me one by one, and I could accept them and send the payment info it would be great!