There are two routes to purchasing your own piece of virtual real estate. You can either start from scratch and build from the ground up, or you can purchase an already built home on the web and continue its legacy in your own style.
From the Ground Up
The true pioneer; I applaud you. When you purchase an unsettled domain name there is a lot of work to do – more than most people imagine. There is content to publish, personal connections to make, back links to build, marketing to be done, and careful attention needs to be paid to the way in which you brand yourself on the web. You have to figure out how to create a buzz about your idea, product or service. You have to dedicate the time and energy to creating a website of value, and building a community of readers that will both spread the word about what you’re doing and participate in your community. You need to constantly be honing your vision for the site, A/B testing different strategies, and keeping detailed logs of what is working vs. what is scaring people away.
Buying Pre-Owned
As anyone searching for a home can attest to, it is very rare that you find a place that already has everything you’re looking for. Certain sacrifices will most likely need to be made if you’re buying an already built home. Whether it be giving up on that extra full bath or settling for a 1 car garage instead of 2, or maybe even going with an entirely different style of home all together. Sometimes you get lucky. You may be in the market just as your dream home appears, and the same can happen with domains. Every so often a domain name with an already established web presence within a particular niche will become available. In the same way your newly purchased home will be devoid of furniture and food in the fridge, your newly acquired domain will no longer have any of the information it once had. There will be no images, no logo, no pages/posts, and no guarantee that people will still continue to come to the site after the change. You will still have to add your own content, ideas, designs and energy to the domain, but at least there is a solid starting point. This established domain will come with some back links, ranking and a presence which would take you months (even years) to create on your own.
Ethics
The ethics of moving into a previously settled domain… There are some factions on the web that believe it is unethical to take over a domain. To them I ask this question – is it unethical to move into someone’s home after purchasing it at a foreclosure sale? Of course it isn’t. That person knowingly discontinued their payment on that piece of property, and as a result of stopping the payments, the property was re-sold to another party. Now, I agree it would be wrong (not to mention completely creepy) if the new owners of the property moved in and pretended to be the previous owners, but fundamentally there is no issue with transferring ownership of any sort of property, be it virtual or physical. It happens all the time. What is the difference between a real estate investor purchasing an oceanfront home on a prime lot and a web developer purchasing a domain with good page rank and a substantial amount of back links? Is the real estate agent supposed to build his own home in a not yet established town simply because he knows the market? Is the web developer required to build a site from nothing simply because they know how?
Intention
In the end it comes down to intention. If your intention is to purchase an established domain and act as though the site never changed hands, you will not be a success (think creepy people moving into the foreclosed house). However, if you are honest with the site’s existing audience regarding who you are, and what your investment in the site is, then you can proceed with a clear conscience toward success.
I also want to take this moment to remind everyone that the internet isn’t this nameless, faceless, and vapid nothingness we often perceive it to be. Behind each site (ok, not every site, but MOST sites readers find appealing) are geniune, real humans looking to share their ideas, help others and be part of something. While only sticks and stones have the ability to break bones, names can seriously hurt you in the age of the web. All we can do is try to be our best each and every day, and after a solid effort today, I am headed to bed 
Happy Domaining!