When I was selling the packaging would depend on the item.
Most cheap cards were -penny sleeve/toploader/two pieces of cardboard then into a bubble envelope. That's the stuff under about $20. No DC at first, then when they started click and ship it was included cheap so it was a good deal. No insurance, I figured if something cheap got lost or wrecked it was easier to just refund and move on.
More expensive stuff I usually went for priority in the small flat rate box. Or the smaller one before the flat rate. Those will fit inside a flat rate envelope if the rate seems too much. Sometimes I'd just pay the extra if the final price was a surprise. Most of the stuff had cost very little. I didn't just do cards. I'd also usually pick up the insurance on those nice surprises.
The plate I sold for 600+ got bubble wrap inside a tiny crate I made for it inside a bigger box with either loads of bubble wrap or packing peanuts and some reinforcements. I paid the insurance, it was only a couple dollars. (I'd paid .25, and yes, I reported the profit.
)
The few times I've sent stuff to SGC I used the priority boxes.
I only had one thing briefly go missing between 98 and 2009. A reel of old movie film that hadn't been used. Packed pretty well, but not crazy -it was in a metal can. About a month after sending it I got a questionnaire from the PO since they'd found some stuff and thought It might be from me or to me. A couple weeks later they sent me back half the address label on a shred of bubble wrap. So I emailed the buyer. I was surprised he hadn't let me know it didn't arrive. To my surprise it had arrived. On time!
In the can with the other part of the address label taped to it inside one of those "sorry the machine ate this" bags.
Steve B