The design corresponds to the features of shape, ornament, pattern and configuration of the Real Estate Sign Post shown in the attached drawings, wherein : FIG. 1A is a front perspective view of the real estate sign post in an extended position; FIG. 1B is a front perspective view of the real estate sign post in a retracted position; FIG. 2 is a top view thereof in the extended position; FIG. 3 is a back view thereof in the extended position; FIG. 4 is a bottom view thereof in the extended position; FIG. 5 is a left side view thereof in the extended position; FIG. 6 is front view thereof in the extended position; FIG. 7 is a right side view thereof in the extended position; FIG. 8 is a top view thereof in the retracted position; FIG. 9 is a back view thereof in the retracted position; FIG. 10 is a bottom view thereof in the retracted position; FIG. 11 is a left side view thereof in the retracted position; FIG. 12 is a front view thereof in the retracted position; FIG. 13 is a right side view thereof in the retracted position; The stippled lines in the Figures do not form part of the design.