Selling things online is easier than ever. Standing out to shoppers is getting harder.

Kevin Stecko has spent more than two decades selling nostalgic apparel emblazoned with He-Man, ThunderCats and more online at 80sTees.com. But lately, he said, some customers seem to have a hard time finding him. In Google searches for terms like “He-Man shirts,” he said, his site appears beneath paid ads from competitors.

His novel solution: a paper catalog mailed to thousands of homes.

“The print cost plus the postage costs should actually do as well or better than it does to acquire customers online,” Mr. Stecko said. The catalog costs about $86,000 to produce, though he is waiting for more sales to come in before judging its success.

The tech ecosystem that powers ecommerce has simplified the way aspiring merchants set up shop, from web design and inventory management to email marketing and sales-tax collection. Shopify Inc., a provider of such services, said 1.75 million merchants used its platform last year, more than double the number two years earlier. But the lower entry barriers and rising cost of online advertising are making it harder for existing and new sellers to cut through the crowd and find more customers.