Some suggestions:
1. Create an account at usps.com. With that you can have delivered to your door, free of charge, a large variety of USPS boxes and envelopes suitable for shipping using their Priority Mail service. With that account, you can also schedule them to pick-up packages, again,free of charge. You can also reuse Amazon boxes and packing materials. Purchase of commercial boxes can be avoided.
2. With an inexpensive digital scale and cloth tape/tape measure, you can weigh and measure your outgoing packages and print you own postage labels assuming you have a printer at home. With 1 + 2, you can ship everything without going to the post office, UPS store, etc. Use PayPal's shipping website and you can print shipping labels at a discount: paypal.com/shiplabel/create/. You can also print postage using your USPS.com account, but only for Priority Mail.
3. Regardless of where you list your items, have plenty of photos of each item, and identify flaws--i.e., what you would want to know if you were the buyer. Doing so avoids post-sale disputes.
4. Regardless of where you sell, research your items' values using eBay's "sold" and "completed listings" search features.
5. Understand that between eBay and PayPal fees, they will take about ~15% off the top of your sale price. Avoidance of eBay fees (~10%) is a significant benefit of selling on the forum.
6, Many buyers rely on PayPal's buyer protections, so having a PP account for your buyers to pay you with makes sense.
I am no expert, and what I've listed is not exhaustive. But I do sell items here, on eBay and elsewhere from time to time, and they have served me well.