Well your opponent was not really a sandbagger, his moves 3-3 invasion and such was not joseki and left many weaknesses behind:


The reason why you lost is because you need a bit more training in: basic life&death, basic shapes and other basic stuff.
I really recommend you to read "Attack and Defense" from the Elementary Go Series.
So after you start reading it you will realize what you need to change and then you can win against this player

I think he is plain dead. I don't see the ko.

Thanks for showing me this! A lot of times I actualy can solve stuff in tsumegos pretty fast but I seem to forget to check status of groups (including my own groups) while playing.

Not sure I understand your advice, do you mean i should read attack and defence in order to fix the basic stuff or is that just to give me general middle game direction? I will read attack and defence anyway though, this was on my todo list already. Or are you sugestion I should dive into tsumego/tesuji in order to get better at life&death and shapes.

Or should I just play a lot of game to improve these basics ?

Please elaborate how you would improve these "basics".

Thanks a lot!

Reading Attack and Defence fix a lot of basic stuff and give you a direction how to attack and defend.
To tell the truth "Opening Theory Made Easy" and "Attack and Defence" are the only books I read to reach KGS 8k.
Opening Theory Made Easy is a good book that explains a lot of basics, I strongly recommend to read both books.

I think you can start to solve life&death and tesuji problems, but you should first learn "eye shapes".
For life&death I recommend the books "1001 Life and Death Problems", "Graded Go Problems for Beginners Vol. 1-3".
For tesuji I went through "Tesuji" of elementary go series.

And the last step is posting a serious game for review in the "Game Analysis", best are close games or with good examples of fighting.
I think I posted one game each week until I was KGS 10k and that helped a lot.

Edit: @cyclops: You are right, no ko, black is dead if white plays 'a'