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You could do what I've done. My shop changes what it sells fairly often. First off I sell what I have an excess of and do not need (depending on the item, sometimes at a very marked down price to get rid of it). If I see things that sell pretty well then I try to keep them in stock like I used to do with wood before people started buying wood at crazy high prices. Don't be set on just specific items, if you find something that sells well, keep it in stock either by collecting it when you are running low or by setting aside some chests at your home or storage area for excess of the item to make restocking easy. Something to look at doing is to establish a buy/sell shop. I don't recommend doing this until you either have over 1 million coins to absorb the mass selling that will occur when you first place those chest shops or to start with all of your buy/sell chests full. The advantages of a buy/sell shop is you do not have to worry about filling up those chests as much as people will sell to your shop and other people will buy from your shop. A quick example of this would be my buy/sell name tag chest shop. People can sell me name tags at 5000 coins each and buy them at 6000 coins each. So for each name tag that is sold to me and then bought from me I make 1000 coins, and yes they do sell, just not super fast. I know I didn't give you any advice on what to really sell, just some overall pointers from what I've seen you have to do to make your shop successful over the long term, and just remember not to get discouraged if it takes you a while to make back the money you spent on the shop. It took me about 2 1/2 months to make back the money I spent on the shop, others it takes longer.The last bit of advice on a shop I will give you is to watch chat and see what players are asking for and if you start to notice people asking for a specific item or items, then that would be something good to sell in your shop, either as a short term temporary item to make some quick money or as something you stock up on and sell for the long term.
Iron Blocks- selling 64 for 6000, buying 1 for 3000