Kaidee.com will soon start charging a posting fee for customers wanting their ads to appear in a prime spot on the online classifieds website.
The move, set for year-end, is part of efforts to generate fee revenue in the coming years after offering a free service for users.
"The online classified ad market for second-hand goods is still at an early stage, and it needs to convince users to make use of their unused or old products," chief executive and founder Tiwa York said.
He said in Europe, online classified ad sites were ranked in the top five most popular, but here they were in the top 20.
TNS Research International (Thailand) estimates the country has 12 billion baht worth of unused items in good condition that can be sold on the internet.
The country has at least 12 million potential sellers to advertise their second-hand products online.
Kaidee plans to boost its number of sellers by two-thirds to 2 million by year-end from 1.2 million last year.
To meet that goal, it recently started broadcasting TV commercials to reach the mass market, particularly upcountry.
Currently, half its users are from Bangkok and major provinces such as Chiang Mai and Chon Buri.
Last month, Kaidee introduced a new chat menu to allow sellers and buyers to discuss the products.
The site also encouraged consumers to use its mobile app to post their products for sale.
It hopes to convince more than half its customers to use the app next year, up from 17% of 8 million visitors a month now.
The company found the top six product categories were used cars, amulets, motorcycles, mobiles/tablets, property and the grouping of fashion, shoes and bags.
Some 54% of its users are male, mostly over 20. The impetus for sales are competitive prices, honest, quality pictures and an attractive description.
The website has a new office covering 1,500 square metres to serve future growth.
It was designed under the "Eat, Play, Work" concept to encourage staff creativity and collaboration, which is embedded in its corporate culture.
Kaidee is 56% owned by 701Search Co and 44% owned by Naspers' OLX.
The website was rebranded from Olx.co.th in January after the two companies set up a joint venture.
The joint venture came after Naspers, Schibsted Media Group, Telenor Group and Singapore Press Holdings (SPH) established a joint venture for developing their online classified platforms in the four major markets of Brazil, Indonesia, Bangladesh and Thailand.
Majority owner 701Search is an equal joint venture between Schibsted, Telenor and SPH.