Running a sale with no professional help: If you've never run an estate sale, it will pay to get a professional to run it for you. Or at the least, hire one to advise you.
Under advertising: You can't over advertise for an estate sale. At the least you need to put it online, post fliers, post it in your newspaper classified section. And another good reason for #1 above, hiring a professional, is that they have a regular following and usually a killer client list.
Marking every tiny little thing with a price tag: It always amazes me that so many feel the need to price every single menial item with a price tag. Want to assure your buyers you don't know what you're doing? Price each individual avon bottle at 50 cents each. You'll be marked as prey immediately.
Having no, or poor security: Think there's not much theft at these types of sales? Think again. That gold jewelry and those silver morgan dollars will disappear if they're neglected.
Poor display: I'm not talking here just about visual display, but practical display as well. Will the crowd be able to get through the maze of aunt Mildred's glassware to get to the furniture? And yes, if your ads are good and you have quality in the sale, you'll have a CROWD.
No parking plan.
Not checking permit requirements. Did you pull a permit if you need one? Some towns, such as Rockport, MA, require a permit. Some residences such as Condos, do not allow any type of on-site sale.
Not making the grounds safe. You're likely liable for any accidents that happen.
Not planning dispersal of left-overs. There's bound to be a lot of un-sold items. This is another area where the pros earn their money!
Not having an early bird plan. If you don't know what this means and why it's important, you're in over your head.
Not knowing enough about what you have. We've all heard the stories about amazing treasures that were found at an estate sale for a give-a-way price. They're true, at least some of them are. Again, if you don't want hire a professional to run the sale, hire an auctioneer or antiques dealer for a few hours to tell you about what you have. It'll be money well spent!