But you might have trouble with immigration, see my post above. And Air France or Lufthansa or KLM or Swizz Air: good luck!! A normal clerk at the counter: does not know that you can extent. And immigration in Thailand does not need to grant the extension.
The simple solution is a ticket where you can change the return date, if the fee is not to high. One way flights out of Thailand are usually more expensive than one way flights into it. The other solution of course is a cancelable ticket, as some people mentioned already or a cheap bus or train ticket to for example Laos. Laos canceled the "visa on arrival" (which costed $40) and you get now a free 14 day entry stamp: "Visa Exempt".
To make it straight, I do not know why even moderators do not know that stuff.
It is not the airline to bother with. It is the immigration officer at the border that gives you the stamp: to bother with. It is Thai law to have an onward ticket. Hence: it is mentioned as a requirement for your visa.
The airline "might" be asking because they have to ship you home on their cost if you get entry denied because of a missing ticket.
But: as many people mentioned, you do not need a visa anymore. As the entry stamp is 60 days and can be extended ONCE for another 30 days.
The proof of onward travel can be a bus ticket out of country. E.g. from Mukkdahan to Savannakhet. Does not need to be a flight. Savvanakhet is nice town, especially after August. Perhaps you want to visit it anyway?
And before the nitpickers come and say: "you never get asked", I got asked by Air France clerks once in Paris, when I had booked a flight with Air Oman. When the Oman clerks heard my troubles, they gave me a return ticket for free, with a smile and comment: "This is a gift from the Sultan of Oman" and a second smile: "please do not use it".
I once got asked on Suvarnabhumi airport on arrival by the immigration officer. When I fiddled with my iPad (which was switched off during flight), he said: "okey no problem"
There are other options, like refundable flight tickets. But: do not cancel it before you are in the country.
I saw enough people who got seriously rejected entry because the officer said: "I do not care that you have a refundable ticket. But trying to insult my intelligence, by coming here with a pdf of a ticket that is already canceled: I care"
I do not know why this attitude pops up so often in Thai Visa Advice forums: it is not airline issue. It is Thai law. And the Airlines follow Thai law. On top of flying you home on their cost (which they will recover from you later), they get a hefty fine! (And that fine they will recover from you as well).
For running a restaurant you will have some problems, e.g. simply getting Greek style Yoghurt (yes you can get) and cheese from goat or sheep. Or even good olive oil. Sent me a PM, I can give some hints.
You just cross the border. And come back. There is no trick in it. No agent needed. You can cross a land border 2x a year. Rumors are the 2x limit was dropped, I did not check that.
You not need an agent o simply follow a simple law.
Your answer. What you think why people are here? Because they do not comprehend the links google gives them. They need/want answers in lay men terms. Trying to turn one away from a group which specifically made to exactly answer such questions: is rude.
Is it income? Then technically yes. Do you seriously believe they care? If you have income outside of Thailand: why are you not volunteeringly taxing it in Thailand, instead of in the other country?