95 civic dx 1.5 liter MAP sensor

I'm missing my MAP sensor. My car ran fine on Friday and today, Monday it would'nt run. I've figured out my MAP sensor is missing. 3 questions- 1, can MAP sensors fall off or is it more likely that it was tampered with? 2- is it easy to replace myself? 3- should i spend the $300 the mecanics say the part costs or buy one on E-bay for $40??

thank you for your help. here are 2 pics of my missing part...[img:681e716362]http://www.2carpros.com/forum/automotive_pictures/99163_100_1597_2.jpg[/img:681e716362]
[img:681e716362]http://www.2carpros.com/forum/automotive_pictures/99163_100_1599_2.jpg[/img:681e716362]