Get that thing proofread. Get yourself a few beta-readers, have someone besides your girlfriend, your mother, or your sister read it – someone you can count on to tell you the truth.
And seriously, get it proofread. If someone looks at the sample chapters on Amazon and starts counting the number of errors per paragraph, they’re not going to buy it. It won’t matter how good the story is if people can’t even bear to try to read it. If you can’t afford the bucks to pay someone, you may be able to work out a barter of some kind, a trade, a cut of the profits, something – but always remember, you get what you pay for.