I have bought a number of bikes into Thailand over the years. New and used. Tourist visa and spouse visa. They are all still in Thailand. I never declared anything - just put through Emirates oversize baggage. Only got stopped once with a very large bike box. The customs guy asked 'What is it?'. I said 'a bike', an he waved me on.
Bought lots of tools through, a big trolley jack (that was a struggle), hi-fi system, race exhaust system for a motorbike etc. Customs have never shown much interest.