shipping to Thailand is offered by some Shops from overseas, but you can't have an Ebay Account or Ebay shop in Thailand. In fact, there's no Ebay in Thailand. What are you smoking?
Try to avoid DMK for the return. There are some Pitbulls @ Immigration. With 5 month you should be fine. If there are more than 180 days of stay within 1 year you might run in trouble and being sent back to Vietnam. All up to the IO.
For foreigners: you don't need to enter your passport number to log in, you need to enter 13-digit ID which you get from the hospital when you got your jabs.
The DLT won't accept the docs later this year anymore, if sold by foreigner. Next to sales contract and green book there's the certificate of residence neccessary. That's valid only 1 month.
Give it all to some moto-shop and they will do it for some bucks tomorrow.