I think one problem is that a lot of people are expecting too much money for things.
There are t-shirts that I have for sale in my shop in Melbourne that I might have priced at $40 or so. Someone else in Australia (not a dealer) will try listing the same thing, in worse condition for $200+ online. People see prices from high end sellers (who have earned their reputation to sell things at high end prices) and list their similar item for the same price.
This especially doesn't work when you factor in exchange rates.
Realistically, if something is worth $100USD in The States we should only expect $100AUD or less if we sell it here. People see an item priced at $100USD so will price their's at $135AUD but it won't sell for that.
Of course, there are exceptions to the rule when something is super rare or desirable.
None of this may apply to you but I have been studying the Australian eBay vintage t-shirt market for many years and these are things I've picked up on.
If you start auctions low, you will almost always get what they are WORTH.
Contact me if you have any questions.
I might be able to help you work out which things are worth your time to sell online.