Jennifer Cline of Monroe, Mich., has agreed to sell the document for $7,000, autograph dealer Gary Zimet told the New York Post.
Cline, 28, is "selling the letter for a down payment on a house and to pay off medical bills from her cancer treatment," said Zimet, who listed Obama's letter on his website, momentsintime.com.
"Handwritten letters of any sitting president on White House letterhead are extremely rare. It is certainly worth more than I am paying for it," Zimet said.
Cline wrote to Obama in December, telling him of getting laid off from her job as a pharmacy technician in 2007. "I lost my job, my health benefits and my self-worth in a matter of five days," she wrote in her three-page letter on lined notebook paper.
Cline said her boyfriend (now her husband) lost his business, and she was diagnosed with two forms of skin cancer. She struggled as a single mother without health insurance.
Obama, who is said to regularly read 10 letters from the public a day, was shown Cline's on Jan. 8. On an undated White House card, he wrote: "Thanks for the very kind and inspiring letter. I know times are tough, but knowing there are folks out there like you and your husband gives me confidence that things will keep getting better!"
Zimet said he got the letter on Saturday.
"The letter is a historical document, and it is very hard for her to part with it," Zimet told the Post. "It's very timely considering the elections. But I don't think she's disillusioned with Obama -- this is just about surviving and practicality. She is selling it to pay for a house, which I think is poetic justice."
"I don't really know why I wrote it," she told the Detroit Free Press in March. "It just made me feel good at the time."
She told the paper she's written to every president since being assigned the task in second grade but had never received a response.
Cline and the White House did not get back to the Post. There was no phone listing for Cline.