Buying a bike sight unseen is a risky proposition at best. Easier to travel to where the bike is, complete transfer at the nearest LTO, only when that is complete fork over the money and buy it, and then post it if you don't want to ride it home. Also as Tony said, some offices won't accept a transfer on a tourist visa and the ones I have been to also wanted the letter from your appropriate immigration office which verifies your address and status to put it in your name. Without the letter and appropriate Type O visa the LTO's I've dealt with (Khon Kaen, Surin, and Roi Et) would NOT allow the transfer! So protect yourself, if not as they say a fool and his money are soon parted, especially here!
Good Luck! Brian