Financial terms of the transaction are not being disclosed.
In a press release, Linnamae said "UP Invest will use the money to fuel expansion of other group companies, including media businesses in the Baltics, and look for new and exciting investment opportunities in other fields".
Apax Partners said it was their first investment in the Baltic countries.
"We look forward to investing in BCG further, with the support of our Operational Excellence Practice, to enhance the great value these marketplaces provide for advertisers and consumers," Tom Hall, partner at Apax Partners, said in a press release.
"We are confident in the growth prospects of the region and have been very impressed with its business friendliness, pragmatism and entrepreneurial culture," he added.
Justinas Simkus will continue as CEO of BCG.
In Lithuania, the company owns the online classifieds platforms Aruodas.lt, Autoplius.lt, Cvbankas.lt, Kainos.lt, Paslaugos.lt and Skelbiu.lt.
The media organizations owned by Linnamae-controlled businesses include 15min.lt. BNS, the Estonian daily Postimees and the Latvian online news site TV Net.