Looks like lowpro got it pretty much covered, if you want to make sure the buyer is serious you can ask for non-refundable deposit in case the buyer needs some time to gather the cash or something. If you are not selling a high priced motorcycle I don't need you need to bother with any written agreement, your text conversation is good enough, and the best way is to both go to JPJ together and do the name change so you are sure all is settled.

If the bike's road tax is dead/expired for more than 3 years then you will need to have inspection done first.

Happy selling!