I'm against the other two postings actually
In many ways, reselling someone elses VPS service that they're already known for can be a great way of kick starting your entry into the VPS world. Think of it like this;
You mark up the VPS pricing by 10-30% (whatever you wanted), pick up full managed VPS machines, and then resell it to your client. You can then choose to do the work yourself, or better yet, when a client enters something to your helpdesk, if you don't have a quick answer, or need extra guidance, contact your reseller location.
Reselling another established hosts VPS services can be a very easy entry. It'll give you time to learn all the various aspects too. At the same time, if you're working with an established company, you can utilize multiple machines at their location. This can help offset your SINGLE VPS machine going offline, and all your clients contacting you at the same time and threatening to leave while you attempt to figure out the problem.
From an initial startup point of view, being able to diversify over multiple machines can be a huge benefit.
Having your own machine will allow you to charge your own markup, and possibly undercut the competition. Be aware that one machine going offline can cripple your business model within a few hours. (this is true of all hosting reselling though)