I hate selling cars. If I buy a Honda or Toyota would they Gove me a trade in discount on a different brand? Or would it be so low as not to matter?

Dealers don't usually take trade ins in Thailand, but rather refer you to an independent that buys used cars. My GF got a pretty good price on her used 7 year old MItsubishi Mirage (135K baht) but the guy who bought it was referred by a cousin. Apparently many of these small cars are sent up to Issan area as there is a shortage of small affordable used cars, musch less choice than in BKK area. I truthfully expected only about 60-70K for that car so pleasant surprise. We asked at the Mazda dealer and they referred a guy who called and offered 100K if I remember. 

 

So ask the dealer to have someone call you and offer a price and see if it fits in with what you need. Otherwise can place an ad..but more hassle.

It is absolutely worth asking. I know the Isuzu dealer I got four pickups from the guys there were always trying to buy the old ones.

 

I think they quick-flipped them to the used car dealers. Take it to a used dealer and have them give you a price before you go to the new car dealer.  

Yes, they would lay off the vehicle to an independant (as stated earlier) which gives you/the dealer a starting point for negotiation. Any further compromise would be defined by the value of your new purchase and their need to meet sales targets.