What is Reddit's opinion of Metronic Poly Mailers 12x15.5 100 Pack Large Shipping Bags，Strong Adhesive Mailing Bags, Waterproof and Tear-Proof Multipurpose Envelopes for Clothing, Small Business,Boutique, Postal Bags Blue?

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Honestly you'll figure it out as you go but here are some of my tips to help you get rolling:

1. Unique items are going to sell. They may not sell fast but they will sell. This can be holographic sports shoes, obnoxiously bright windbreakers, creepy dolls, you name it. There is literally a Ebay category for weird stuff (link).
2. Find a niche or category and specialize in it. When you get into a groove, learn another.
3. To see if an item sells, check up the name and go to completed/sold (on Ebay) and this will tell you if a lot sell. Always look at how recent they sell as well which leads me into my next point.
4. Don't stock up just on longer hold if you have limited cash. When you see that they consistently sell but not often, it's okay to pass up on it.
5. Nike is super easy to resell because all their products have codes that you can check up unlike other sports brands like Under Armour or adidas. It is normally two letters the a couple numbers like this "CI4465-341". The last 3 numbers indicates the color of the item.
6. Keep an Excel sheet of what you put up, when you listed it, all the costs involved, etc. This helps give you data on what sells well and what doesn't. It'll also keep you from accidently accepting offers that are too low. I do it pretty casually as this is my layout.
7. Don't skimp on the packaging. People like to know that you actually care but don't go crazy with it either. I use these for smaller clothing (which comes to .13 each) and these for books (about .48 each). They look clean and professional.
8. If you are looking for places to start. I say sport clothing and shoes at Ross. Books at Goodwill. Odd items and toys at garage sales. Craigslist or FB Marketplace is a hit or miss for just about anything. I got the whole Harry Potter set (hardcovers) for $25 but I couldn't bring myself to resell them.
9. USPS is going to almost always be cheaper for smaller and lighter items. Heavier items, go through UPS. Don't overlook USPS media mail either. Hope these help!