I have sold three properties in Bangkok. No problem and the same goes for buying if you do your homework.
There is one BIG advantage in buying secondhand and it is worth considering. If you buy property in a moobahn that is already three or more years old you at least get to see the worst picture since everyone will have done their extensions etc. Some of these can be real eyesores. Any new built village looks good when it opens for sale. But once the owners move in there is no telling what they will do to their property. I have seen properties painted completely purple or pink whatever including all doors etc !! Hideous walls and awnings etc. So....secondhand you can escape the fate of having the new owner next to your new house turning his into a slum that reflects badly on your place..
In addition , when buying secondhand you get a good idea of who your neighbors are. Do you want to live next door to someone who has three pitbulls or a champion crow in the front garden. Would you like live next to a party animal who invites his friends around to sit in the garden and play loud music ? Stuff like this can happen when you buy new, It is a lottery. Sure , when they sell you your new house they will tell you there are rules and regulations that owners have to abide by. In reality , enforcing these rules can be very lax indeed.
Above all....don't rush. Take your time and look at as many places as possible and try to talk to as many people as you can in the places you like to make sure your big investment doesn't turn into a nightmare.