There are a lot of mistakes sellers make that could cost them a sale, but I had a unique one a few years ago that not only cost a sale multiple times for my seller, but it also cost him $28000. The house was a simple plain vanilla ranch house. The local market was just in the beginnings of the 2008 market plunge. So, the comps on his house came in at the pre-plunge prices, and it was a pretty nice range.
We listed at $140000. Immediately, I received an offer at $130000. Based on where the market was heading, that was an outstanding offer. The seller came back with a counter at $138000. The buyer countered at $135000. I'm yelling, "Take the money and run!" But, my seller decided that this was a game that he liked to play. He came back at $135500, and the buyer walked. He was tired of the game. He gave up a great sale for $500.
After that, it sat for nearly a year without a buyer. Finally, I talked him into dropping the price to $130000. Immediately, another buyer came by with a good offer. He offered $125000. Again, I begged the seller to take the money and run, but nooooo, the game was afoot. He came back at $128000. The buyer didn't even bother to respond. He just walked. I dropped the listing at the end of the listing period. He ultimately sold the property for $112000. His propensity to negotiate cost him $28000.
Sellers can do a great deal of harm to a sale by getting into that mindset. Everything doesn't have to be a negotiation if the offer is a good one. This is where an agent may be able to offer advice, but not always. I brought current sales to show him. I showed him where the market had gone during the listing. I showed him the difference between houses selling for more than his vanilla house, but nothing would persuade him to use good sense. He just had to negotiate. It cost him plenty.