The SAFE Act enacted in Oregon in 2010 has made owner-carry contracts very difficult for both the buyer and the seller. Basically, you must qualify, as the burden is on the seller to prove they collected documents from you indicating you can afford to purchase the real property. If you cannot and run into problems later on, the seller would be liable to you to terminate the contract and pay back all monies you paid on the purchase.
Your best bet would be to work on your credit score and get your debt-to-income ratio in line so that you can qualify through a lending institution. It's actually not as hard as you think. Go to creditkarma.com and pull your free credit report. They will give you pointers about how to raise your score.