EBay warned last night that profit this quarter could fall below analysts' estimates as it invests in marketing and a revamped website to attract more shoppers.
This sent its shares down more than 5% in after-hours trade on Wall Street.
The online marketplace is making a big push to catch up to Amazon.com with three-day guaranteed delivery and a more user-friendly website, hoping to distinguish itself as a haven for specialty items rather than commodity products.
It said marketing expenses rose by nearly 5% in the third quarter from a year earlier, as it got that message across to potential customers.
Factoring in the higher costs, eBay forecast fourth-quarter adjusted profit, excluding some costs, of between 57 cents and 59 cents per share.
On that basis, analysts on average were expecting a profit of 59 cents per share, according to Thomson Reuters.
For all of 2017, the company narrowed its forecast for adjusted profit and said it now expects $1.99 to $2.01 per share.
In a sign the investments may be paying off, gross merchandise volume (GMV) - the value of goods sold on eBay websites - rose 8% to $21.7 billion in the just-ended third quarter.
That is the fastest growth eBay has reported in three years.
"Our customers are responding to the significant product enhancements we have been making, and this is reflected in our results," the company's chief executive Devin Wenig said in a statement.
Revenue in the quarter increased 8.7% to $2.41 billion, edging past analysts' estimate of $2.37 billion. Net income rose 27% to $523m.
Despite this growth, the California-based company said its operating profit margin decreased slightly to 24% in the quarter, from 24.4% a year earlier.
EBay reported nearly 2 million more active buyers in the third quarter to reach 168 million, excluding customers in India.
Active buyers tallied at 171 million in the second quarter, before eBay sold its India business to Flipkart in August.