Hey john
i have a 23l oak barrel i use for beer. Sorry but i wont sell it. There kind of hard to come by used and they defietly have a shelf life. You'll probably only get 3 years of oak character tops. I got mine from the oakanagon barrel works in oliver bc. Id suggest a few things if your going to try to use a small barrelfor beer.
-Try only using the same recipe.
-Have a lot of beer waiting to go in the barrel. Like 15 gallons or so.
-Only put beer in it thats a higher gravity.
-Top up with carbonated beer that you bottled from the same batch or recipe. And bung with an airlock so it blows of the oxygen
-top it up every few days with a small barrel or itll get air exposed

hope that helps a bit. I wouldnt buy a used one for beer, once you dry out the barrel its ruined. And if someone is selling an old 5 gallon wine barrel they probably have used up the oak flavor and are selling you an infected barrel

brad