You have a feedback score of 92. Majority of buyers will see that and that is good enough for them, especially if it says 100% positive. Most will only go looking if you are less than 100%.
Put your prices up and don't go into debt for no reason. If you want to offer free postage, that's fine, but you need to incorporate that cost into the purchase price. Someone has to pay for it, and that someone is you. You also have to consider packaging costs too, whether it be boxes or padded bags.
Unfortunately, you probably won't do well with phone cases, for the reasons already mentioned. You also have to consider that phones are being updated all the time, so if you've bought 10,000 phone cases for the latest iPhone, they are going to be worthless in 2 years and you could be stuck with all this product you can't sell.
Unless you have outlaid a bucket load of money already, ditch the idea of phone cases. It is even more saturated than pre loved women's clothing, and believe me, that's saturated. If you sell in a saturated market, you are going to be very disappointed.
And get rid of the stolen photos. You're just telling potential buyers where they can go to get the items for the same cost as you. Take your own photos so buyers know what they are getting.
