I buy on a mix of both. 3balls and ebay are my choice places to purchase used clubs. Clubs are so pricey I have a hard time buying new tbh. Last year I bought new g5 iron set (black dot) right off the rack. Last year I also got to play 50 rounds, this year I am on pace to do that much again. The previous years I played 10 rounds or less.
Spending a lot of time playing now I felt I wanted to make some changes to at least try and have my score reflect the amount of time I play/practice. One lesson and the instructor makes note that all of my irons are getting heel scuffs. Last week I cleaned them up and have now gone another week of play, heel scuffs again. Today I bought a used g2 iron set on ebay (orange dot). Thankfully they were under $200 and are fairly comparable to the g5 I have gotten a little used to.
I'll be tracking myself with great detail over the next month. I'll be sure and update what I've seen in these changes. Fitted clubs are the to go, so that's how I am going. I do not regret buying new off the shelf irons, I just wish the "pro" at Golf Galaxy had spent more than 10 minutes with me to be sure I got what I needed. Slightly used g5 black dot irons soon to be listed on ebay, watch for the listing if interested =P