Your baffle is not working. Try another baffle, and do not put in more food than the birds can finish in a day (make sure the feeder is empty at night).

Trap the possum with a Havahart cage trap #1078 or #1079 depending on how large. You can then relocate or otherwise dispose of the seed stealer.

I would not trap and relocate the animal. In most cases, the animal will die. It seems to me to be a very human-centered way to deal with a problem that we create. The opposum is simply doing what animals do. Since they don't have grocery stores; they recognize a new food source and exploit it. I think there are probably 10 more possums that will come along when that one is gone. If it isn't a possum, it will be a raccoon or other animal.

In the short term, you could think about these things: Is the location of the feeder in an area where the oppossum can jump from something? If it isn't near anything, you may need a different kind of baffle. You could try the kind they put on bird boxes that wobble when the animal jumps onto it. I'll link below to an inexpensive plan for one you can make. You could also take the feeder down at night.

In the long term, you could try planting for the birds. I understand your desire to see the birds and feed them, but natural foods will give you the same effect and provide more for the animals. Artificial feeders concentrate birds to one area encouraging predators (hawks, cats, etc.) They also help to spread disease like finch-eye conjunctivitis. If you do plant for them, then you are also providing more habitat, places to nest, hide from predators and eat. Try googling up habitat for the birds you are trying to attract and you will find lots of lists of plants and other aspects of good habitat that you can provide. Good luck to you.

Here is a link that might be useful: birdhouse baffle

It is a bad idea to trap an animal that is being attracted to the food you put out. Especially possums.

1. Possums are nomadic, so it is likely there are different animals eating the seed you leave them.

2. Not removing the seed will attract more animals, so if you trap one, more will follow.

3. Trapped and relocated mammals often die after relocation because they are removed from their home territory and thrown into other territories claimed by other mammals. It is not recommended by the Humane Society of the US nor by state fish and wildlife agencies. Some states forbid it because of the chances of getting scratched or bitten by a rabies vector.

So: bad idea all around. Better to remove the food source attracting the animal. Then there is no problem.