For the most part, scrap metal is a fairly benign material to sell. You gather it (legally), maybe it sits for a while in storage while you build up a stash, and then you take it to a scrap yard to see what money you can get for it. Of course, as with anything, there will be required safety precautions. For scrap metal, these can sometimes seem alarming to new sellers because if the safety is ignored there are some serious consequences. However, dealing with these issues is a lot simpler than they may indicate at first.
Cuts and Tetanus Boosters
Likely the most frequent issue with scrap metal is injury in the form of cuts, and since you're dealing with rusty metal a good portion of the time (or at least surroundings with rusty metal, even if you don't take the rusty pieces yourself), you really need to be current with your tetanus boosters. These work in a 5- to 10-year window; for the first 5 years after getting a booster shot, you don't need to return for another one if you cut yourself or sustain a puncture wound. (You do need to treat the wound properly.)
However, during the second 5-year period (years 6-10), you need another booster if you cut yourself on something dirty or rusty. The World Health Organization says you don't need another booster if the wound is clean, but given that pathogens are generally invisible, and you never know just how clean the cutting item was, you should go back for a booster anyway. If you think the cost of the shot may be too expensive and your insurance won't cover it, you can get more affordable shots at many pharmacies. For people without insurance, county public health departments often have vaccine clinics.
Soil Pathogens and Pollutants
Agricultural and garden- or lawncare-related scrap metal carries the risk of soilborne pathogen transference. In other words, there could be some dirt with a garden fungus or bacteria on it that the transport of the metal then spreads to other areas. When transporting metals like this, clean off as much dirt as you can and then protect your vehicle and storage area from residual dirt with tarps, bags, and the like. Monitor any greenery around your property for signs of disease, and treat that immediately.
Radiation -- No Joke
Admittedly, radiation-contaminated scrap metal is not something you're going to encounter very often; many people will never encounter it. However, if you do come across scrap metal from salvaged medical devices, for example, there is a slight risk. And while the amounts might seem small, they can be enough to ruin an entire batch of metal and require decontamination of a scrap yard. To avoid this issue with confidence, know the origins of the metal you collect. Do not try to take apart medical devices, and do contact your state's radiation control office.
As you can see, these issues are not that difficult to avoid or mitigate. With proper care and monitoring you can find scrap metal that is safe to sell, and you can keep yourself safe from any issues the metal might bring.