I personally am not a fan of New Kids On The Block, but I know that a lot of people are and have been waiting for a reunion. Boy and Girl bands of the 90s and early 2000s were a big hit, and a lot of people have been looking forward to reunions for all of them, especially older adults who don't like recent music and would prefer a blast from the past.
Whenever a band "reunites", there is a frenzy to get tickets to see the next happening on the concert agenda. Music lovers and fans of the New Kids On The Block are plentiful and like most reunions, people like to be a part of the nostalgia that comes along with this. It is an evening of great music for their fans and a night to remember forever. The sell-out is a sure thing for a night like this.
New Kids on the Block were a big deal back in their heyday; times have changed and people are ready to move on. Personally, I think it's more hype than anything; people are having nostalgic thoughts on how cool they used to be and now they want to re-live their childhood. Even record sales can have an impact here; compare their 1990 CD to their 2013 release and it's not even close.
The reunion will probably get some people attending, but not many. It has been too long for it to still be popular. I personally don't know anyone still interested in New Kids On The Block. I think that the reunion might not even turn a profit, due to low attendance.
I don't think New Kids On The Block tickets will sell out. Although most people are huge fans of Justin Timberlake, they are less likely to go to a group show than they are his solo show. The audience that listened to NKOTB in their prime are now at a point in their lives where more important things matter. They will have a lot of sales, but I doubt they will sell out quickly.