For those who have used ebay

I plan on using ebay to purchase a used android. It's my first time using ebay. I have a 100$ ebay giftcard. I just have a question. I just set up my paypal account. If I am going to just use my giftcard to pay for the entire total cost of the phone, do I still need to link my back account or credit card to the account? Also, since you can't really just add the card to your paypal balance, you can only type it in during checkout, am I going to be able to "make an offer" on some ebay listings since there is no funds currently in the account? And lastly, are you able to use more than one giftcard to purchase one item? Because I have 2 50$ giftcards.
thank you