It also depends if you told him you were keeping them forever, or if u would keep them for a year or two and then u might flip them like u might do to all your pins, but if he knows u generally flip machines, then sell the machines, make the profit, and throw some of the profits back towards your mate.
If he wanted those machines back, he would have mentioned it to you. If u promised to keep them forever which is why he passed them to you, u should halve the profits.

He was generous to start with which gave u a good deal. The new buyer thinks its a good deal as well as he is paying the money to acquire those pins from you.

If you dont at least offer your mate something as a gesture of good faith, he may never do pinball business again with you.
I believe in karma, what goes around, comes around. Friendship is worth more than money.