Is there much of an advantage of using a domain purchasing service through your registrar vs. and independent domain buying service?
I'm assuming he means independent domain and domain brokerage services,
Basically these domains tend to be shorter (not always) and keyword rich (not always).
Also generally these domains are aged and may even have backlinks to them before which in these days can be damaging if they are not relevant to your own niche but with this said I can't see few harming your set if they have been linking for years - it may even help.
Your find the likes of GoDaddy are also domain sales that aged and premium in terms of keywords of the length of the domain.
Personally you should go for a branding that you want rather than an expensive domains. Google has clamped down on keyword rich domains and its now better to think about branding, or least thats what I believe. Its not a bad thing being different and having a memorable brand is it? :P
With a pure domain service provider you often get more possibilities to configure your domain, such as DNS settings to route your domain (and any A-Records, CNAME, etc.) to another webspace / server / nameserver or IP. You also should be able to define the WHOIS info of your domain.