Weeeeell, if you really want to argue that there was detrimental reliance, I suppose you could make a legal case in small claims court, but you don't want to resort to that. If you do custom orders, it's best to get payment up front, or at least a deposit. When I custom design from scratch, I do a deposit first just in case I can't design what the person wants. I put that up as a reserved listing and don't start work on the custom order until the deposit is paid.
Regarding reserving items already made, if you're only sales opportunities were in that little window of time, you've got bigger problems than the fact that you reserved for that time. You've got a business niche that just isn't workable in that case.