I believe that buying used is generally preferable, mostly because you want interconnects or speaker cables that have a lot of hours on them and are thus fully broken-in. Stated another way, most copper cables require 100-200 hours of break-in to really start to open up, and cables with silver conductors can take forever to break-in. Thus, buying used can get you around the painful break-in process and gets you a cable with hours on it (the rule is, the more hours, the merrier). Of course, you have to hope that sellers know and will be candid with you regarding how many hours are on the cables, and frequently, they are not, because they think that labeling a cable as "new" or "low hours" will increase opportunities for sales (it does, to those who do not know any better).
Exceptions to this general rule may include flat cables like Nordost and Goertz, which are in my experience prone to getting damaged over time because of their flat design (but if the pair is mint, no problem).
As for brands, you need to find out what works best with McIntosh equipment, as cabling is extremely dependent upon synergy with specific components (I've never owned McIntosh gear and have no recommendations). Cables from Cardas, Kimber, Audioquest, Jena and many others are competently designed and manufactured, and can sound great in a given set up -- you just need to try to find out what works well with McIntosh gear.
If someone enthusiastically recommends a specific cable to you on Audiogon, make sure to check their equipment for sale to determine whether they have a conflict of interest.