So if I have a game already on Google Play and I want to sell it on Flippa what's the best way to do it:
Whatever game you sell that is connected to your Google Play account is going to be transferred to the buyer. Google has a process for this and I suggest you follow it because it's easier and faster in the end.
Whether or not you reskin before or after the sale is entirely up to you. That's more of a business strategy in my opinion that should be considered if the game is in demand. Test it first. Sell one version and then decide based on your feedback whether or not you can make ongoing sales.
You can resell the same game on Flippa multiple times.