That's the PRAM battery holder. The plastic cover is probably broken or missing. That's what the opening above the power switch is for. Sorry to disappoint you, though.
Depending on the condition of the model (no case cracks, scratches, how much RAM installed, if software is included, functionality, etc.) I'd say that 35 euros may be a bit high. Here in the U.S., Macintosh Pluses sell for near $0 dollars, no matter what condition they are in. The model with the starter minimum value for selling would be the SE/30 or Classic II. Does it include a keyboard, mouse and software? If so, then all together that will make the 35 euros worth while.
73s de Phreakout. :rambo: