The ornamental design for a baseseat of a ball chair, as shown and described.
FIG. 1 is a perspective view of a base seat of a ball chair showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof; and,
FIG. 4 is a perspective view showing the design in use, in which the base seat is installed with a back and wheels of a ball chair.
The broken lines shown in FIGS. 1-4 depict environmental structure and portions of the base seat of a ball chair that form no part of the claimed design.