May be its simple problem but I am not clear. What if I make more than 1 account from same ip and bookmark my sites there? will they ban? or will that negetively affect?
If they spot a pattern then it will look unnatural and they will know what is going on. You have to make it look natural.
Depends on how it is done. I don't know how good at identifying patterns and finding out about this stuff del.icio.us is, but I know digg is pretty good at it, so it would not surprise me if del.icio.us is also. You may want to check out this thread on DP members banned from digg. http://forums.digitalpoint.com/showthread.php?t=105564
A great way to get others using del.icio.us is to go through gangclick.com. Take a look if you get a chance. You can also get people to stumble and digg your site as well.
Is anyone still using Digg / del.icio.us for legitimate uses? I think those sites are both being pretty heavily gamed at this point. StumbleUpon seems marginally better though (traffic referred from there seems to hang around a bit longer in my experience).