Koulibaly has established himself as one of the best and most in-demand defenders in the world since joining Napoli from Genk in 2014.
The 28-year-old Senegal international was heavily linked to Manchester United before the Premier League giants signed Harry Maguire in a world-record transfer prior to the transfer window closing.
Koulibaly continues to be linked to United and when asked about Napoli's centre-back, De Laurentiis said: "I loved Koulibaly the man so I refused €105million, but a time will come when I'll have to sell him. Whoever says we didn't improve our defence is wrong."
Napoli midfielder Fabian Ruiz is also attracting plenty of interest amid links to LaLiga pair Barcelona and Real Madrid.
Fabian swapped Real Betis for Napoli in a €30m deal in 2018-19 and the Spaniard has starred in Serie A.
"Fabian's always been a top player," De Laurentiis said. "If I convince myself to pay a stranger €30m, I do it for a reason and now I know why.
"It's one thing to play for your country and another to do it for a team who play every three days. We found Fabian, but we'll find more like him. The door's always open. I don't get attached to players."
The futures of Dries Mertens and Jose Callejon are also uncertain, with the Napoli duo linked with moves to the Chinese Super League.
"I'm absolutely not prepared to make a major effort for them," De Laurentiis added. "Each player has a value, depending on where they play, how they play, how old they are and what they do.
"If they want to sell themselves out in China, overpaid to live a life of s*** for two or three years, it's their problem. I can't consider China to be competitive. China's far away. If they consider it to be close, it's their problem.
"In life, you have to choose whether to be happy and find a job you enjoy doing or work just for money. For me, money is a means and not an end; if it's an end for them then they should go to China."
De Laurentiis also said Napoli captain Lorenzo Insigne must "change his attitude" as he threw his support behind head coach Carlo Ancelotti.
Amid reports of a tense relationship between Insigne and Ancelotti, De Laurentiis added: "I didn't fight with Ancelotti, he can stay here for another 10 years. In cinema, my relationships, like my one with [director Carlo] Verdone, last a long time.
"Insigne's an excellent sportsman, but he must stay calm, change his attitude and lay off certain things. He's always had an attitude of discomfort in Naples. I understand him, I protect him and I like him a lot, but he's always found his situation in Naples to be uncomfortable. Therefore, I just want to say that he needs to calm down and become a more peaceful person, but that's his problem. Neither [agent Mino] Raiola nor Ancelotti can resolve that.
"He's a great player and he can be in good form or bad form. If he's less so, it's up to the coach not to play him. Insigne can't come out cracking jokes or with an 'almost' defiant attitude. The coach is a family man, 60 years old, and won't tell you to get lost because he's three times your age."