I actually don't think accepting zero-confirmations transactions is that terrible a thing to do for small volume trades. Let me explain a bit.
Almost every business accepts credit cards. If I pay by credit card, you can see the transaction has been made within a few seconds, but it takes 2-3 days before my card issuer actually transfers the money to the business in question. In those intervening days, it is fairly easy for me to cancel the transaction by phoning my card company/bank and claiming my card was stolen/lost/cloned/phished/hacked/whatever. Indeed, even after the money has been transferred, I can still challenge it and have it reversed if I'm persistent enough. Yet fraud rates are very low and almost every business accepts credit cards.
With bitcoin, the situation isn't that different. You can see the transaction has been made within a few seconds. It usually takes in the order of minutes to hours to confirm, based on your fees, rather than days it takes with credit cards. Once it has confirmed, I can't challenge it, and I certainly can't reverse it months down the line.
there are additional caveats with bitcoin.
at a low enough fee rate, some transactions may never be confirmed, or could take several days after re-broadcasting. so a business may need to incur additional costs using CPFP to reliably receive certain payments within a reasonable time frame (assuming this works reliably at all given miner pool behavior). they may need to require a minimum fee rate or otherwise charge a fee on top that covers their costs to sweep. allowing zero conf tx simply invites people to pay low/no fees, which is problematic for merchants.
the ability to double spend unconfirmed transactions also raises some concerns. with credit cards, your identity is known to the issuer, and there are various factors like IP address/physical delivery address in online purchases and security cameras/witnesses in physical purchases to deter fraud attempts. but with bitcoin, what
exactly constitutes provable fraud since outputs are tied to keys, not people? let's say i buy a big screen tv at best buy, pay with bitcoin with a low fee, walk out into the parking lot then double spend the tx with a higher fee. best buy doesn't know who i am, and to boot, is it provable in court that i'm the only holder of those private keys? private keys are hacked all the time, plus they can be controlled by multiple parties.
it seems like confirmed bitcoin transactions are incredibly secure for merchants. unconfirmed transactions? not so much. this is another reason why flexa's model (and presumably bakkt's) are attractive to merchants rather than accepting bitcoin directly. they don't need to wait for confirmations and accounts can be settled quickly because only confirmed outputs (in gemini's custody) are spendable at POS.