Clay might be a good item to trade as well. It's easy to come by, but a pain to get in any large quantity, plus river bees make it.
You'll be swimming in clay if you use Factorization ore processing. I was able to give away 1024 stacks the other day yet I still have 60k blocks to do something with.
If the server has equivalent exchange then minium shards/stones might be a good thing to sell.
 
				
		 The way it works with the currency is that you can right click a sign which is over a chest and you pay however much $ in exchange for the item in the chest. I'm thinking I'll have people buy some sort of special currency to use in the shop so that they can use say.... an automatic solar panel crafting line. As CC is temporarily disabled though, making my ideal shop is kind of.... Complicated
 The way it works with the currency is that you can right click a sign which is over a chest and you pay however much $ in exchange for the item in the chest. I'm thinking I'll have people buy some sort of special currency to use in the shop so that they can use say.... an automatic solar panel crafting line. As CC is temporarily disabled though, making my ideal shop is kind of.... Complicated