That is correct. Do it by the locker. Here's what I do from buying to pricing and selling.
1. When I get that glimpse at a locker I try to be in the last few people as by then the auctioneer has been pushing people along and then as you are near last you actually get to stand there in front of the unit as the auction proceeds. This gives you added time to look at it and evaluate what you can see, That can make the difference in winning or losing by getting a better longer look.
2. I do my add ups of what value I can see from the door then take that number times 2.5 to arrive at the max I'll go. The 2,5 number is two times what I can see and .5 for a fudge factor so if I see 100 at the door my max bid would be 250.
3. Then after I win the locker I actually look to set my minimum sales value at 3 times the winning bid value to cover costs such as gas, cleaning fees, buyers premiums, dump fees and ebay fees etc etc. and PROFIT. So I'd be looking to get 750 minimum out of a locker I bought for 250.
That means I'd set my sales prices for the sellable items (in total) such that I'd get 750 or more out of it. If what I get hits or exceeds that 750 mark the locker was profitable and I made money, if not I reevaluate what I did in step 2 above for the next locker. Of course there is that factor of luck as you really don't know what is in the locker until you buy it.
Also as others have said you may only get 3 lockers in any given month and 15 the next, coupled with all the back end work to list and sell what you got and how long all that takes is so much of a moving target that it's nearly impossible to base earnings on $$ per day or week. Have I confused everyone yet? LOL.
What you need is three months worth of earnings cash in the bank that you can ride on thru the dry spells and that amount of money is over and above the locker buying funds. Now there are a lot of people who will take exception to what I do and say here and thats fine, this is just what I did and do and it is working. In the last year my plan has worked to the point where (with the exception of health insurance) I can nearly leave my full time job and do this full time. I sell on CL, ebay, ebay classifieds, etsy and others and also now have THREE booths at a local antique mall that my wife and I operate and stock from lockers, auctions, estate sales etc. It has been a long tough road but the rewards are coming more and more.