Gosh that is terribly sad, and trust me I know how you feel after just losing Sparky, also a tumor which as Alyssa said sounds like thats what was wrong with Mark, almost 100% sure on that. And in 99.9% of cases there is nothing to be done, so don't feel bad a vet would have told you nothing could be done so there is no worries.
I wish you reconsider getting a new bird, the way you must think about it is like this. You gave Mark a great life, took him from a bin where he has a very dull life, and you made his life the best it could be, and you must know that Mark would want you to do that for one of his brothers or sisters, give them a life like Mark had, there is no better way to make Mark proud than by taking in a new keet and giving it a wonderful life. No bird will ever replace Mark, but they all have their own personalities and are unique and all are equally great birds. So I know he would want you to honor him by helping another bird to have a great life. I know Sparky wanted that from us, and my dad didn't want another bird and he basically had talked my mom into not getting one, but I pressed the issue and soon she was thinking about it but wanted to wait a couple weeks, but then I said I went and looked and saw some very good birds and the next day she went just to look and came home with a bird. She is so happy she did and so am I. I know the loss is extremely hard to deal with and accept and it will hurt for a while, but you are honoring Mark by getting a new bird, he would be so happy you were kind enough to take another birds life and make it great. SO I wish you would think about it, and definately stick around here, we enjoy having you and even if you don't have a bird we would enjoy your company.
Well I wish you the best in life and I hope you come back very quick and also get a new baby bird soon, you will be very happy once you do. From the minute you bring him home you will be so happy and know that it was a good idea. Anyways thanks for sharing and we will miss you very much until you come back to us. Best wishes. Jloc