Trade is always voluntary.
Trade is absolutely not always voluntary. People need resources to survive, and they are essentially punished for needing them. We need food, and we have to pay for it. If we don't pay for our food, we have to pay for the raw materials, and land isn't exactly cheap. We need water. We pay for that, too. We have to have shelter, and that requires us to either own land and build our own (after buying the raw materials, which again is not cheap,) or rent/buy from someone else. If you own a house within city limits, you have to have water (incl. garbage collection and sewer), electricity, and gas, or face stiff penalties, including having your children taken from you. If you aren't independently wealthy, you have to have a job to pay for all this stuff. A large portion of valid employment places in the United States will not hire you if you don't have "reliable transportation", read: a car (of which even a beat up, used one can easily cost more than $1000), and most places won't hire you without a driver's license (which, surprise surprise, costs money.) This is, of course, due to the public transport infrastructure of most of the US being a sad, pitiful joke. Then, just to get back and forth to work, you have to pay for gasoline (electric cars are routinely above $30,000) and vehicle maintenance. At the very least, you'll be spending money on the parts to fix the damned thing yourself when it breaks down.
Let's say, hypothetically, you find a patch of dirt on the outskirts of a small town where you can bike everywhere. It's unlikely, but it could happen. You could gather materials from the wild outside town and build a house with your own two hands, right? Wrong. "The wilds" outside town is government property, and collecting resources from it is considered theft. So, you have to buy your own materials. Fine. You put up your house, brick by brick. You do the dangerous roofing yourself, and that's no big deal. However, when it comes to electricity, water, and gas, the connections coming into your house, as well as any shutoff valves and breaker boxes, have to be done by a certified Master Tradesman, or your house will fail the inspection that you have to pay for. So, that's going to cost money.
So, I suppose you could homestead, out in the middle of fucking nowhere, with no way to pay for the resources you need to survive, no hookups to water, gas and electricity, and party like it's the 17th century where you most likely won't survive the winter. Oh, and you have to have a viable (read: conforms to the city's zoning laws, with some minor modifications) free-standing structure built within a year. Good luck. You could take up with the Amish, but they won't let you stay in their villages, help you with your farm, or let you buy land if you aren't willing to attend their church and follow all of their rules (do you like medicine, powered hand tools, fiction books, alcohol, a clean shaven face, colored fabrics and premarital sex? Well, not anymore, you don't.)
Or, you could go homeless in a big city, and sleep in parks and alleys (when you're not being assaulted by other homeless people, being shot at randomly or mugged by gang members, or being harassed by the police,) panhandle and do odd jobs for a little spare cash to the people generous enough to actually pay you, and beg for food. You could shower in truck stops and face possible sexual assault (male or female, no difference) and disgusting diseases every time you take your clothes off. Be prepared for overloaded homeless shelters to turn you away if you're not a pregnant female, and religious soup kitchens to do the same if you're not prepared to join their congregation. Be prepared for the state to try to strip you of any familial ties you have, taking your children and forcing them into foster care, and refusing to let more than one homeless person be on a case for state aid at a time. Be prepared to be denied job after job when you put "N/A" under address on your application forms. Be prepared to fight for every scrap of food you come across, and every cent you beg for. Be prepared to dress your own wounds, or do countless hours of community service for daring to step foot (or be dragged) into an emergency room with no money.
Plain and simple, the deck is stacked against the poorest of the poor, because we have to pay for it with everything we've got.