Consumer checkout is the easy half. The hard half is everything a business buyer expects and a consumer never sees: negotiated prices, credit terms, approval chains, quotes that get revised four times, and an invoice that has to match a purchase order raised three weeks earlier.
Multi-tenant storefronts with per-client catalogues and configuration. Quote-to-cash from first enquiry through revisions, acceptance, order, fulfilment and invoice. Customer-specific pricing and price lists. Buying groups and shared catalogues. Bulk listing, bulk pricing and bulk product operations. Branded buyer portals on their own subdomains. Loyalty, rewards and subscription billing. Marketplace settlement where sellers are paid directly.
Price is not a column. It is a function of who is asking, how much they are buying, what they have agreed, and which group they belong to. Systems that store a single price per product get rebuilt within eighteen months.
A quote is not a draft order. It has revisions, an expiry, partial acceptance, and a history someone will need to produce in a dispute. Modelling it as an order with a status field loses all of that.
The buyer is a company, not a person. Multiple users, different permissions, approval thresholds, and someone who can see prices but not place orders. Consumer auth models do not stretch to cover it.
White-label leaks in the places you did not check. Cache keys, sequential IDs, file storage paths, search indexes, transactional email and error reports all cross tenant boundaries long after the queries have been made safe.
If you settle to sellers, you are in payments. The moment money flows through you to someone else, scheme rules apply, and the structure matters more than the contracts.
If you are building B2B commerce, running a marketplace, or your pricing model has outgrown the system holding it, we have built this several times over.