There are ways a travel agent can get around this, but often they are caught and fined (or given a "debit memo") so few will do it.

It was common practice in the "old days", but then so was using B2Bs.

One last thing: depending on united.com in expert mode is not always a consistent experience: I've seen it show S1 and then give an error when trying to book "Due to pricing fluctuations and availability, that fare is no longer available" or somesuch.

Then I start over, see S1, try to book and get the same error. Then call UA to find out that it's actually S0 and the website is just confused (I know.. shocking... united.com is confused).