It may not actually be a tumor, it looks to me like an abcess. It may need to be drained by a vet, but it does look at bit raw too, maybe put some triple antibiotic on it for now and see how it goes till you can get her to a vet. If you feel you need to euthanize her, you can use vinegar and baking soda to make CO2 and it will do the job. Do this in a sealed chamber.

Call your local humane officer and report Petland, if the other rats they have are any example of what I am seeing in this one, they are being cruel and can be punished for this. I would get pics of the rats you just purchased from them for your humane officer also.

Also, please re-size your pics, they are causing this thread to have to be viewed goofy and over scrolling.