Can someone help me how to get online job which is related to sales/mobiles? I'm anxiously needing it for a part time job...

Be very cautious in your search. Most are bogus. If anyone/company wants money up front, move on to the next one.

There is really no such job, sorry.
Think about it. How would it work?
There are a very few people who are allowed to be CSRs from home after establishing themselves as thoroughly knowledgeable about a company, usually a few years.

Most mobiles are sold off web pages, no people selling them,

There are some telemarketing people, in fields like insurance, and ticket sales, but most companies you will deal with, want you to pay them and no jobs appear.

Sorry but there are few jobs like this.

(I have made plenty selling on eBay, mostly my own stuff around the house, but also for other people. There is a long learning period, and you can lose a lot of money if you aren't studious and alert to changes.)

I have a buddy that does the same thing. It's mostly bike parts for the very serious bikers.