I recently bought 50mm f1.8 lens online, and quality is very poor as described in most of the reviews. But when i shake it gently I can feel some of the parts moving inside. And when ever I slide the AF/MF switch I get same feeling. Is it normal for a new lens ?
I've got a copy of this lens: I can confirm that by (gently) shaking it you can feel something moving (which is the moving part of the focusing mechanism). Apart from a click, no noises are heard when switching the AF/MF lever.
My lens has been perfectly functioning until now (I hope that this gentle shaking didn't affect it ;-) ).
It's impossible to answer this question fully without examining the lens in person. All lenses have some degree of play in the system, and the 50 f/1.8 is well known to be a very cheaply built lens. I can't say whether the degree of movement you're detecting is out of line without knowing what your basis for comparison is.
Do the images meet your expectations, and do they seem in line with the reviews? If so then I wouldn't worry about it.