On an order book, the depth you see is a promise. The market maker showing you a bid can pull it in the same block you try to take it, and nothing on the screen tells you in advance which levels will hold.
On-chain books make that answerable in a way no off-chain exchange can. A resting order has an Order.owner field, and that field is readable by anyone. So if the owner is a contract with no cancel function, the order can't be withdrawn — and you can prove it before you trade against it.
The obvious way to check is one opcode:
if (owner.code.length > 0) {
// it's a contract, not a wallet — this quote is firm
