Can I enter Thailand with a one-way ticket using only a Certificate of Entry (COE) as an American traveler for a marriage visa?

I did

Buy a open return but why a marriage visa why not a retirement visa no need to go out every 3 months.and other benefits.

If your not sure when you want to leave. You can book a ticket with

In the past I used bus tickets. I am pretty sure that is not currently an option. The issue you are likely to have is with the airlines. They sometimes will screen passengers at check-in/boarding to make sure you have all the necessary paperwork to enter the destination country.

I **think** you are not required to have onward travel if you have visa. It maybe the case you need to have onward travel for Visa Exempt (sometimes erroneously called visa on arrival).

If I felt like taking a risk and being cheap, I would show at the airport without onward travel. If I get stopped I would buy a cheap ticket from BKK to anywhere. Currently Singapore $100 from BKK (round trip). You wont use the ticket - so it doesn't need to be round trip or anyplace you want to visit.

If you shop around a bit, you maybe able to find an refundable option. Like if you buy from AirAsia - then sometime in the future you can change destination (from singapore to someplace local).

There are no direct flights from BKK to USA. It maybe worth while shop around for a ticket from any place to USA. Then buy cheap ticket from BKK to anyplace, with options to reschedule, refund, etc.

It all depends on what visa you get to enter Thailand on.

Just coming on visa exemption plus one 30 day extension wont give you much time to sort out bank account and have it there for two months

You can, but you can also get a marriage non O from your embassy showing funds in your bank account there. This would be 90 days and give you the time to get your money in bank account for the two months before you appky for extension.