Some simple tips for profitable trading on the market. (Note to readers, depending on how many people read and apply this will alter whether or not the tips are still useful or not)
Tip #1 Offering to buy a resource at a certain price is almost always a better deal than trying to trade someone else for what you want. However, the drawback to this is if you need a lot of that resource you need to make it a pretty good deal or you will only have a few takers, or if you need a lot of different resources then you are stuck behind the eight ball with that 10min timer for each different trade offered.
Tip #2 The more trades being offered (size of scroll bar in trade window is very small) the better the deals to be had. This is the best time to buy. However, the best deals will always be for coins, so if you aren't stocked up on coins then you had best shop wisely.
Tip #3 Price's you will be able to buy or sell at with a regular number of people online. Thus, if Tip #2 is in effect, these prices will generally be lower in most categories, although a few (random) resources prices will be up because of either a lack of supply or huge demand. Prices are in coins, because trades for coins are almost always available, to ascertain whether or not a trade of resource for resource is fair (i.e. fir logs for tools), convert each unit to coins and see if they are roughly equal.
Prices are given in the range that is both a decent deal and generally offered, try to buy at low end and sell at high end obviously.
Per Stack
These trades are available frequently with small variation in price.
Fir Logs: 2-4 coins
Fir Planks: 2-4 coins
Stone: 2-4 coins
Fish: 2-3 coins
Water: 1-2 coins
Coal: 7-9 coins
Copper: 8-12 coins
These trades are only slightly less frequent but with greater variation.
Wheat: 7-11 coins
Brew: 8-12 coins
Flour: 7-10 coins
Bronze: 14-18 coins
B. Swords: 16-22 coins
Tools: 14-18 coins
Bows: 10-16 coins
Hw Logs: 6-10 coins
Hw Planks: 12-17 coins
Marble: 15-20 coins
Iron: 20-26 coins
Horses: 20-26 coins
These trades are offered some of the time with a large price variation.
Iron Bars: 35-60 coins
I. Swords: 95-115 coins
Gold Ore: 70-100 coins
Steel: 150-200 coins
St. Swords: 200-230 coins
Sausage: 27-42 coins
Longbows: 45-68 coins
Meat: 16-30 coins
Gold Bars: 150-200 coins
Trades for items of expert level are rare to see although prices are pretty consistent.
Titanium ore, Granite, Ew logs, Ew planks all trade for very close to 1:1 resource to gold coin.
The other items in expert level you don't need a guide for if you are able to use them because you have been playing forever already.
You will find that you can sometimes find items for cheaper than I listed, and also sell higher than I listed as well. This guide is just to give you an estimate, the fun part is finding your own trading niche where you can buy low and sell high and get those coins or resources you need to build up your island and conquer those bandits and adventures!!
Hope it was helpful.
Edit### Just because I'm tired of seeing people try and rip newb's off, in the PROVISION HOUSE, you can convert 250 fw planks into 50 hw planks, or 250 stone into 50 marble. SO TRADING FOR MARBLE OR HW PLANKS AT MORE THAN A 5:1 ratio of basic for advance is A RIPOFF. DON'T DO IT!!!