Jeni Sandberg usually deals in vintage and collectible items, not in hot new stocks. Still, the home-based art appraiser and consultant plans to take a stake in Etsy Inc. when shares in the online marketplace go public this week.

Ms. Sandberg, who lives in Raleigh, N.C., has been a vendor on Etsy for five years, earning income from her sales there and from work as an art consultant. A former specialist at auction house Christie’s, she manages her own investments and is “by no means a massive player in the financial market.”

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