The ornamental design for an accessoryseat for a buggy, as shown and described.
FIG. 1 is a rear perspective view of an accessory seat for a buggy according to the present invention.
FIG. 2 is a front-left perspective view thereof.
FIG. 3 is a front-right perspective view thereof.
FIG. 4 is a front-bottom perspective view thereof.
FIG. 5 is a top plan view thereof.
FIG. 6 is a front elevational view thereof.
FIG. 7 is a left side elevational view thereof, the right side being a mirror image; and,
FIG. 8 is a rear elevational view thereof.
The broken line showing of support structure beneath the seat and the wheel is included for the purposes of illustrating environment and forms no part of the claimed design.