I have not purchased from this guy, but have a few oak barrels that were all "new", but had sat a few years before I got them. This is how I treat them before putting beer in.
With each, they need to be soaked until they do not leak (like I said mine have all been sitting dry for more than a year before I got them... but they have also been much more affordable for what its worth). To jump start the "cleaning" process (if I can submerge the entire barrel in something like a rubbermaid container), I usually add about 1T of washing soda (aka soda ash) per gallon to the soaking water. When the barrel holds water I put it through 1 or two more weekly water changes with soda ash solution. This stuff is a strong base (high pH), so I then follow with a combination of citric (or other food grade) acid and some sort of sulfite solution (e.g. campden tabs).
After a batch of beer I follow a cleaning regimen as above. I can usually do about three batches of clean beer before there are distinct sour characteristics developing. Barrels are not going to keep producing clean oak flavored beers indefinitely, and if this is your goal, you should use chips, cubes, or something other than a barrel. I started experimenting with oak barrels because I want to make sour beers. I usually make a batch or two of "clean" beer with a new barrel, and then begin with flanders and lambic style beers. From what I have read, if your barrel produces vinegar, its firewood.