Welcome to the Forums!
If you can post or private message me the ad for the trailer and whatever information you may already have from the seller, I can look it over and point out any red flags that I see. Otherwise, go to the Portal tab at the top of the page and find the "Buyer's Inspection Checklist" This list will walk you through everything in terms of checking the trailer over.
Now, if your intention is to park it in the back yard and never move it again, there is a fair amount of stuff that you needn't worry too much about, as long as it is safe to tow to your location. Also, how long are you planning on keeping it? Are you concerned about relatively short-term resale value? Is there any chance you will get the camping bug and decide to fix it up and tow it around? Do you want working plumbing (ie., running water and toilet facilities) for your beer cave?