Before I bought one of these, I insist on being able to play the vinyl and hear it first. So that means you have to be local to me. There are way too many mastering decisions that need to be made correctly for vinyl to sound good. And no, that does not mean being able to take something from your iTunes library mastered for your shitty earbuds.
I do not trust some hobbyist to get any of that right to the point of me willing to pay $10. However, if someone with actual experience with mastering had one of these then I'd be interested, but that dubplate will be way more expensive than $10. A pricetag of $10 tells me the person doing it has no clue about anything involved and has priced themselves too cheap. A person with experience would go broke from the time involved to make any money at $10.
Also, 20 records per week? JeebusChristo that's ambitious. Your sales team would kill your margins at $10 per. Because if you're trying to sell and make the vinyl all by your lonesome, just take the money you were going to invest in getting set up and light it on fire. It'll be faster and much less heartache.