Astonishingly most agents couldn't begin to clearly explain their USP at all.
And it goes without saying that if you can't articulate it, it's very unlikely that your prospective clients will be able to understand it (& pick you out in a red sea full of cut-throat competition and a metric-ton garbage of distractions).
Don't make the mistake of thinking that your USP is simply an "advertising" or a “marketing” issue.
While the way you define your USP will ultimately impact your marketing efforts, you shouldn't view your USP narrowly as simply a headline you use on top of your ads.
In reality, it's a much larger Business issue; Your USP sets the strategic direction for your business. It helps you define where your business is going and what you stand for. It paints a vision others would enthusiastically follow.
(E.g. at my company Comfortable Commissions, our vision is to help all our clients earn commissions at a level they are personally comfortable with.)
Every day you will be in contact with prospective clients. Your USP should be in each of your conversations with these people to help turn them into clients.