For example I'm always keeping the cardboard boxes from the stuff I buy. I make some money on the side by selling things I find at the flea market so all that cardboard comes in Handy when packing for shipping. Other stuff I keep, jars. No point in buying new ones to store my spices, I just use the ones I get after finishing whatever was in them. Same goes for plastic containers.
Everything, almost. I couldn't drop a thing because I rarely throw things, though I may not know for the moment how would I use them later. I think that there could always be something I could get from them during an emergency. So I save them from that, because when emergency comes, you wouldn't know what you'd be needing, you wouldn't even know how you'd be dealing it. So, it's always best to be prepared with all that you've got.
I reuse those ice cream plastic containers and I store on them various things. I also keep the jars of sandwich spreads or any jar that has a large, round, opening. I throw away those ketchup bottles though, since I don't see any use for them.
The main thing in my house that I refuse to throw away are plastic shopping bags. I actually use them as my regular trash bags. I have a large can in my pantry but instead of spending the money to buy large bags we use the grocery bags for every day trash and then tie them up and throw them in the larger can. I notice that we actually do save money this way because decent trash bags can actually be quite expensive. Using the little bags also prevents us from over stuffing a large one and having it break. we actually take our can and just dump it straight into our complex's outdoor dumpster.
I save all sorts of things to reuse again. My partner is planning to start making homemade wine so I have been saving wine bottles. I save some jars for keeping things in and formula milk tins for putting turps in when I am decorating. I save carboard boxes, like you, for the things I sell online.
I try to reuse everything. I keep good sized jars and use fancy soap dispensers and decant other products into them. I also use the the glass holders from candles and use them as holders for my make up brushes or for toiletries in the bathroom. They're also useful for cutlery and pot pourri in rooms and look stylish. It can be a bit messy getting the candle remnants out, but soak in hot water and they residue all floats off.
I save all kinds of jars and decorative glass, I find that I will find interesting and crafty uses for them throughout the year weather it's making a vase or using them in holding candy for presents around the holidays. They are unique and pretty and definitely a money saver... glass jars and the like tend to be super expensive when buying them retail.
I save and reuse margarine containers as food containers that do not need to be returned to me, because I refuse to lend out my good Rubbermaid containers with the easy find lids.