It's no secret that online retail is surging.

Exchange-traded funds tracking the theme have made major strides this year and are among some of 2020's best-performing funds in the market.

Here are some top e-commerce ETFs' year-to-date gains:

However, concerns are swirling around whether these ETFs or some of their underlying stocks are overbought. Amazon, up over 78% this year, is a top holding in both of ProShares' funds.

"No matter how you slice it, online retail is expensive right now," Dave Nadig, chief investment officer and director of research at ETF Trends and ETF Database, told CNBC's "ETF Edge" on Monday, adding that ONLN's price-earnings ratio was hovering around a sky-high 180 times.

But not all online retail is created equal and some plays in the space remain more attractive than others, Nadig said.

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