Watching an episode of Ruddy Hell, the phrase "I saw you coming" was printed on a shop and the shop owner used it when talking to the rich lady. The context was that he sold stuff second hand and even openly admitted that it was only worth "a fiver" when he bought it. He then exclaims: "But I saw you coming ..." which apparently is the excuse for a much higher price when selling.
Now, I get the approximate meaning from the context, obviously. However, how would native speakers explain the exact meaning of the phrase?