Buying something like a house, a piece of fine art, a used car or a business is as much a marketing exercise as selling that very item might be.
There are two common approaches. The first is to denigrate.
Explain that the seller has bad taste. That the car isn't in good shape. That the art was poorly selected for the resale market. Poke holes in the business model, the management team or the landscaping design.
Better still, make the seller feel as though she's on thin ice. Bring an exploding offer to the table and watch her squirm as it goes down in value from day to day. Point to others that have waited too long to sell and how they ended up regretting it. Question her values and her judgment.
In other words, go for the win, where winning is defined as getting a great price.
There are two problems with this approach. The first, and the biggest, is that anything you truly want to buy probably has multiple buyers interested, and with better information available every day, the best stuff is going to be sold to someone else. Your denigration strategy is going to inevitably limit your pool of available items to sellers with self-esteem or desperation issues.
The second problem is that the word spreads. Your gallery or your buyout fund or your dealership quickly earns a reputation (there's that marketing thing again) as the buyer of last resort, once again creating an environment where your approach determines what's available to you.
The alternative is to respect and to communicate. After all, you're here to buy something–I'm guessing that's because you think it's worth something more than you're willing to pay for it. So value the judgment and taste of the seller. Be clear about what you like about it, be honest about the value that's been created. Even better, instead of coming in high and then figuring out ways to bully and lower your offer, come in low and enjoy the process of bidding it up, making the seller root for you and look forward to hearing from you. (This is particularly useful when making an investment where you want management to be happy with you after the deal is done).
In a fair market, it's entirely likely you'll end up paying precisely what you would have paid using the other method, but you'll be offered more works, more stuff worth paying for, and your reputation will reflect that. Most of all it's important to understand that we're not talking about bushels of wheat. Very little is a commodity, and the method you use to buy your expensive item may be even more important than how much you pay.