Simply put, most professional bettors are average handicappers, but active shoppers, which makes a huge difference in a bettor's bottom line.
Note: I guess my wife would be a great Professional, she shops all the time.
On top of that, the professional bettor will practice solid money management techniques, which also adds to the bottom line, while the recreational bettor is more apt to make money management blunders, which will cost them money over the long haul.
I am really talking about professional gamblers and not most the hacks selling picks. "Those who can, do; those who can't teach".