I would just wait and buy a SIM card from one of the counters at BKK after you exit customs and immigration. As I recall, prices start at 200 or 300 thb. So the price you are seeing on Ebay is about the same as you'd pay in the airport. I've used the airport counters for TRUE or DTAC SIMS many times. It's fast, and they will install the SIM and activate it for you. Since activation, and proper application of credits, can sometimes be a bit of a mystery that varies from country to country, I think its well worth while to have their help. Have your passport ready, they will take a copy in order to meet legal requirements, and cash to pay for the SIM.