Dealing mostly with business owners, these are the top three mistakes Merge sees owners make when they are buying a web site:

1. We just want a web site. No you don’t. You want your web site to do something. A web site needs to help accomplish a company’s business goals / solve problems. A company doesn’t need a web site for the sake of having a web site.
2. We can get it for X amount cheaper. Is your goal to get a web site for the least amount of money or to invest in a web site that will generate the most results.
3. We need it fast. You’ve lived without it this long, why the rush now? Concentrate on doing it right versus getting it quickly. Great web sites take time, and I highly recommend taking the time upfront so you get a great result-producing solution upon completion.

Just, Cheap, Fast. Yes, these are mistakes you’ll want to avoid if your goal is to have a result-generating web site.