Inventory to sell on eBay is closer than you think and it can be free. Look around out in nature for items to sell on eBay - many of these items are used as craft supplies or home decor. Depending on where you live, there are probably highly desirable items within walking distance from your home free for the taking.
Sea shells - Crafters love these for making wreaths, decorating candles, and making ornaments. Sell small shells in lots and larger shells on their own. Make sure you only take what is allowed from your beach. For example, many beaches prohibit removing sand dollars, starfish, and sea oats.
Driftwood - Natural home decor is a popular trend. Driftwood makes beautiful mirrors, wall art, candle holders, and wind chimes. Some lots of driftwood can sell for over $30. For fast and easy shipping, create your listings based on what will fit in a USPS flat rate box. Again, make sure it is ok to pick up driftwood from beaches before taking it home to list on eBay.
Gourds - You probably won't find these growing in the wild, but lots of farmers and country folks grow them, and they are available cheap at country farmer's markets and vegetable stands. Or, maybe consider growing your own if you are a gardener. This lot of 15 gourds sold for $107.
Tumbleweeds - Kansas has tons of tumbleweeds free for the taking. In fact, some people think they are a nuisance. Creative people actually buy these to make crafts, furniture, or interesting home decor. This tumbleweed sold for $16.99 plus $12 shipping.
Pine cones - Both large and small do well, and especially around the holidays when people use them to decorate for Thanksgiving and Christmas. Make sure they are free of dirt and insects before photographing and shipping.
Acorns and acorn caps - Again, these are popular for fall decorations. You can sell the entire acorn with the caps, or the caps alone. A lot of 50 large acorns sells for around $15 plus shipping.
Keep your eyes wide open for what natural resources are available to sell on eBay. Something familiar and plentiful in one area may be scarce and highly desired in another. You can be a supplier for a crafty person who wants exactly what you have in your backyard.