I bought a bike with all paperwork green book etc but the seller has gone overseas and the page with his visa stamp has expired. Can I still transfer the bike? Office says no. I am still in touch with the seller via email.

No you cannot transfer the bike into your name withouth the previous owners visa stamp being up to date. Totally illogical, totally wacko, yep, this is Thailand. It doesn't matter you have a signed bill of sale, you have the bike and you have the log book you still can't do it legally. 

I have a bike with the same problem, I knew it when I bought it, but it's a big bike 750, and you will find that most older big bikes don't even have a log book, at least mine has a book, meaning I can use it, tax it, etc.  

You can do the same. The letter of the law says the vehicle must be registered in your own name, so in effect we are breaking the law, but the law makes it that we can't anyway register in our name, yes, TIT.

I have a contact telephone number of a guy who will do it by the back hander method. It's a risk. I prefer not to take the risk as then he will have my log book while the transfer is being done, and maybe I don't get it back, better to leave things as they are I think.

Perhaps if your seller comes back with a new visa you can do it then.