Why would you want to buy an existing name, rather than make up a new one? The only good reason I know of would be that an existing name is particularly attractive to you.
If so, fine, but you're at the mercy of the particular owner. If you fancy amazon.com, then likely you aren't wealthy enough to buy it.
If some company has bought up a higher level name, and is selling sub-names of that, say I buy up thepythoniccow.com, and am making a business of selling names like grit.thepythoniccow.com, then whether you want to deal with me depends on how bad you fancy that particular name, and on my reputation, and whether we can agree on a price.
But unless there is a particular name you really want that someone else already, I'd recommend you create your own new name, and avoid the hastles of trying to do business with a current name owner.
Play around with names you like at the GoDaddy site, or other DNS provider, and see what is available that strikes your fancy.