I think it depends on where you live and, as others say, how much travel you're willing to do. If you're in a large metropolitan area with a lot of shops, it's not as difficult as in a less-populated area, even though there are more shoppers in the more-populated metro areas. I see a lot of shops in Pa., Md., and the D.C. area that are all outside a range that would make it profitable for me, unless I did an overnight route and picked up several shops. I'm willing to do that, since I don't have young kids anymore, but there never seem to be enough of them in the same general area to make it worthwhile. I was ready to make a 3-hour one-way drive to Virginia to do a $300 audit, but somebody grabbed it before I could. I'm sure I could have found a shop or two on the way down and on the way back!
In my immediate vicinity, there's not enough work to make a lot of $$ shopping, unless I'm willing to do $10 and $12 shops every day, every week. Which I'm not, because I do have other self-employment income. But I'm making more this year than I ever have MS'ing and working less. Jobs here are becoming bonused more often and for higher amounts. I think some shoppers may have dropped out of the field in my area.
My goal is to pay for at least one car payment, which I usually do easily, and half or all of the other one. I made that goal last month and this month. So I'm happy! LOL.
Good luck! I'm sure you can do it with planning and flexibility.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.