Other than consolidator/bulk tickets which are not applicable here, there is nothing a TA can sell you that UA won't sell you directly. If it's complex enough, you may want to spring for the phone booking fee or pay a TA to deal with the hastle.

But, it really isn't that hard. Once you decide what cabin you want, you then want the cheapest fare which allows you the flexibility you need for the date(s) you need.

There may well be seats available in a given class of service, but not at a fare which you consider reasonable. In that case, you either cough up more than you want to pay or move on to the next alternative.

Given your confusion, you may find a [U]good[U] TA well worth it.