In Travian you can trade in two ways, with other players so you swap one resource for another at a stipulated ratio. as an example I would get some ore and trade 100 units for 100 units of grain at a one to one ratio, I could decide to get more so 100 ore for 200 grain but would have far less people interested as they would be going for the 1:1 ratios to get the best deal. The only benefit I could perhaps offer is I could be closer to a player who is desperate for some ore ASAP...

the other way is to trade with NPC's similar in practice to what stooobs suggested but with an additional cost of gold coins.

Perhaps it could instead be that players sell goods on the open market if no player is around, they would have to travel far to get a better price if different sectors had different market prices, this would however be at a risk of piracy... if I had my base in an area that was mining a certain product it would be pointless selling it in the area as so would everyone else hence I get a low price but if I am willing to travel to a farming sector I get a much better price... Perhaps someone would be happy to take the goods off of me for a better price and then sell them somewhere else for a profit so I don't even need to do any dangerous work.