Before committing any crimes, it will be a good idea to unlock a Fence first since no normal shopkeeper will be buying stolen goods from you. A Fence is a dubious shopkeeper that buys such stolen items at a fair price. You can only carry a specific quantity of each type of stolen items so it will be a good idea to be able to lighten up your load by selling them as soon as possible. In addition, you can also buy helpful consumables, masks, and pamphlets (recipes). They can also craft unique accessories that provide permanent bonuses to Arthur.
The first fence you’ll unlock is found in Emerald Ranch, during the story mission The Spines of America. Once unlocked, other fences in major cities like Rhodes and Saint Denis will be available as well. In addition, Seamus, the Emerald Ranch fence is the only guy that will buy hijacked wagons and coaches from you, but more on that later.