Lets look at the benefits.
Allowed 2 additional forum accounts - If you are a business with employees, this is great.
Threads appear on the homepage - amazing advertising bonus here.
Username is highlighted yellow - This in it self is a level of advertising.
Access to reverse IP search - If you have a site you can refer back here to see if a buyer has account here to verify identity or identify in disputes
Allowed 1 affiliate/referral thread - Good for extra profits.
Can access canned replies system. - Mild benefit for quick responses
Less forum ads - Who does not want less ads.
So right of the bat you have a lot more advertising from being a proseller. The yellow color it self stands out and many users only deal with pro sellers for currency and accounts. Its a major level of trust, that someone is willing to invest in their account here. Why? Because you have to earn back what ever you spent and shows you are into growing your business. You have to spend to make money, plain and simple. Not saying you have to spend money here to make money here, but you know what I mean. You have to invest in yourself/business in order to grow.
I do not have a figure for this but I can see it watching activity streams, posts and feedbacks.
A proseller would have to chime in give their side of experience.