I would keep it very simple. Maybe something like this:

[i]Dear X

This is just a note to thank you for taking the time to write an autograph / donating x for my x-auction. I truly appriciate your kind thoughts.

Once again thank you and have a beautiful day,

X.X

[/i]

Could someone be able to help me create a thank you letter that I can send to the celebrities thanking them for donating items