when buying your boxing gear, wether it be gloves, bags or even clothing, the first thing to do is to check out what condition it is in. for things like gloves i prefer for them to be new or only slightly worn as they will provide the best comfort and protection for your hands.
However with bags, it does not matter so much with what condition it is in. generally a new bag will hit the same as a used bag and will still give you the same desired effect ie punching a bag. however you should make sure that there are no splits or tears in the bag as this will make the bag life less. also another thing to check is if it comes with any extras like chains and or hooks as these can be extras needed to be bought which adds to the cost. also make sure that the size of the bag (if buying a punching bag) is the size needed. No use buying a tiny bag if you are a heavy hitter, unless that you want to use it for fast hands.
also when buying in boxing accessories, make sure they are your size and are still of quality value as they would will be used repeatedly.
one final thing is to check out where the product is being shipped from and the payment method. some things that ship from thailand make take longer to get there and would also incur higher shipping costs than from your country. check out then that things in the shop work out to be cheaper or more expensive than what you could get on ebay to see wether or not you would be saving or wasting money.