Here's the FTC's Mail Order Rule:
http://www.business.ftc.gov/documents/bus02-business-guide-m...
This spells out what has to happen when a seller can't deliver on time. There are time limits. Extensions on delivery times require the buyer's explicit consent. If the buyer doesn't do that, a full refund is required. Without the buyer asking for it.
Responsibility to comply with the rule falls on the "seller", the "person soliciting the order". If another party actually ships the product, and they screw up, that's the seller's problem, not the buyer's. So you don't want to set up a platform where you take orders for a third party and give them the money up front. If the stuff isn't delivered, you, the platform operator as seller, have to pay out refunds.