Well, I now know the reason why they were able to keep their belongings despite being reduced to slaves.
The Flaest Family shouldn't be underestimated since the weakest amongst their family members are at 7th Tier Mage while the strongest amongst them is at 2nd Magic Circle.
If they had wanted to, the Flaest Family can resist being enslaved by the Demonia Family but it will risk most of their members from dying so, despite their strength, they didn't resist the Demonia Family from enslaving them.
Because of their power, the Flaest Family wasn't mistreated in the slave market and they were able to keep their belongings which are mostly their clothes.
After entering the underground mansion, I was greeted by one of the members of the Flaest Family as he bows down to me.
"May I ask who is this distinguished guest?"
"I am the master of Elise Von Flaest. Elise and I don't have the master-slave relationship but the master-disciple, while I am the master and Elsie, is my disciple."