There's no way you can be sure that the CC numbers you read out to Dell rep won't be misused by the Dell reps. It sucks that they don't have an online payment gateway (in India) but it is what it is. CVV number is obviously required to charge so they need to have it if you go through CC payment. If you're so worried, pay them via DD. It just lengthens the process by a couple of days at max. BTW, the Dell rep apparently enters your CC details into their system which can't be retrieved (as in **** are entered and recorded so that your CC is charged when the order is processed directly). But when you speak out the numbers, the Dell rep can note down the numbers and (ab)use it by shopping online until the card reaches its credit limit.
Another work around is to create a VCC and give those details to the Dell rep. Again, not all banks provide this. As VCC is for 1-time use only, it's VERY VERY SAFE. Google for more info.