The sensible thing to do with links now is to focus on getting quality ones. The problem is that this is also what your competitors are doing. You need an edge, you need to be one step ahead of your them, you need to be right there are the top of page one. We'll explain here a little known way to gain enormous reputation with the SEs. It's almost guaranteed your competitors aren't using it.

here's a reason why this is at #1. It's the single, most important factor. To state the obvious, you can develop a site, increase its PR, explode the traffic, gain subscribers... but you just can't make it old overnight. Other things being the same most SEs prefer links from mature sites. And, the older the site the older some of its IBLs. It's possible that SEs will also use age of links as part of their algorithms if they aren't already.

A domain registered for several years into the future suggests to SEs that the site intends to stay around for a while and wasn't registered as part of a churn 'n burn plan. There may be an SEO advantage purely in the long registration (extending the registration is something you can do even after you buy a domain/site).

Caveat: When a domain's registration is allowed to expire it loses its history and for all practical purposes is considered a new domain. You can use a good whois service to query the site's continuous existence.

2. Relevance

This could vie with Age in the importance stakes but I didn't put it at #1 for one simple reason. Relevance can change over time. It's a titanic of a beast and requires small corrections in its course (to play it safe with the SEs) but a site about Auto Racing Magazines can gradually be made to look more like a site about Auto Racing Fan Merchandise. As you can see from the DMOZ categories here they share most of the breadcrumb in the URL:

Site buyers usually care about earnings more than they care about any other feature. They're looking to buy sites as going businesses; they're looking to profit directly from them. You're not. You're looking for sites that represent good places to get links from. The ideal site will be one that doesn't require frequent updating and can be left to retire. You don't care if they're not earning any money - you're looking for the sites that most others don't want! And that means you're going to pay very little for them.

A site gets its reputation from incoming links and can easily lose it by linking out to the wrong kind of sites. Google claims that the worst of sites can link to you without damaging your reputation (using the logic that you have no way of controlling what others do) but when you link out to bad neighbourhoods it does damage your credibility. Xenu linksleuth is a free and invaluable tool to analyse a site's links. Running a Xenu check on the site, analysing the results carefully, and following some of those links can reveal a lot more than just broken links and links to bad neighbourhoods. For example: Too many links out and any future links you add to your main site is diluted in importance.