banished
Sometimes I can't trade bigger amounts of goods because of a limit.
What kind of limit is this?
Do I have to comply with an overall value?
Or is there something like a capacity (dimension, weight) limit?
(Are the boats that small as the boat footprint and rower may indicate?)
Or is there some limitation in the mixture of goods?
Citizens need a source of food and warmth to survive. Since the Help Menu states that citzens will only visit the nearest storage barn to collect resource then you can't depend on citizens going to your primary settlement to get resources. Even if they do take the long walk back to your primary settlement they will be wasting a lot of time doing so.
Your best bet would be set-up a Market near the offshoot settlement to ensure all the goods the citizens need are close by. Market Vendors will gather resources from Storage Barns and Stockpiles all over the map, so it'll ensure that citizens will always have the resources they require near by. The long travel time between the Market and your primary settlement could still cause some problems though, so you may need to put several vendors in the Market to ensure the Market always has a sufficent number of supplies.
Another option would be to build a gatheres hut, forester and woodcutter. The gathers hut combined with the foreseter replanting trees should ensure they have enough food. The woodcutter and logs provided by the forester should ensure they have enough fuel to keep warm. I suspet they won't have access to clothes with this method, but that will just decrease their productivity. So long as their homes are warm they won't die they should just work less and spend more time indoors where it's warm.
The reason you can't trade your items with the merchant is because they have a trade value of 0. Certain merchants won't accept certain kinds of good for trade, and so the value of those goods will be set to 0 when trading with them. An example of this is herbs, which a lot of merchants won't accept for trade, but food merchants will.
Best Answer
Both above answers are correct, however the major issue here is weight. Weight is an attribute in Banished that determines how much fits in a barn, stockpile, or is able to be carried by a laborer. The storage limit is not on the trade boat, but instead on the storage in your trading post.
The table for weights (and the capacity of storage buildings) is here on the wiki: http://banished-wiki.com/wiki/Weight