What exactly can a customer expect from a "travel agent"?
Of course they can expect their payment will be accepted. What else can they expect? That is the question.
In international travel, you can be expected to be informed of travel document requirements, and for tickets into or through the USA, they are legally required to do so. This is not an option. If in doubt telephone the help line at the US Federal Department of Homeland Security. They will be glad to inform you of what you must, by law, do.
Other than avoiding trouble with the Security Authorities in various countries, you should look after the interests of your customer. In particular, if you book a passenger through an international airport in transit to a third country, you should inform the passenger about transit visas if they are required. Airlines are required to do so. Why would specialist "travel agents" be exempt from this? Perhaps there are legal loopholes.
If a travel agency lets customers get into trouble and then slips through a legal loophole, it is important that the traveling public know it. It is a case of caveat emptor. Unfortunately, most travelers including myself expect help in avoiding such blunders from the ticket seller. Airlines help with this. If a travel agency operating in the modern, internet world does not, they can expect the news to travel, rapidly. It is an increasingly transparent world.
In my own opinion, if a seller sells a ticket from place A to place B via place C, they should inform the buyer of any requirements in transiting the intermediate country. I expect that. I think it is both reasonable and ethical to expect that.