I'm not a lawyer, so this isn't legal advice. For legal advice, you need to consult with a lawyer. However....
If you purchased the house, it's yours. You don't need a previous owner's permission to enter the house or the outbuilding. (From reading your question a few times, I'm guessing that you didn't get the keys to the outbuilding, not the primary residence.) It's your property. You can do as you wish.
As for the items in the outbuilding, that's a slightly different issue. Again, I'm not a lawyer. However, if the items in the outbuilding were not addressed in the sales contract--if the sales contract didn't mention them at all--then those items transferred with the property.
If the items were addressed in the sales contract--let's say it's a lawnmower and the sales contract either said that the mower did convey or did not convey--then you go by the sales contract. That more frequently occurs with things like drapes, lighting fixtures, and appliances.
As a courtesy, you might contact the sellers and request that they remove any items from the outbuilding. However, as I said above, unless the sales contract specifically states that the items did not convey, then you own them and can do with them as you wish.
Again, for legal advice, check with a lawyer.
Hope that helps.
Try www.findlaw.com or www.lawyers.com. I use these websites alot at work, you should be able to use it to pinpoint a good lawyer that can help you in the area you need.