You shouldn’t have to withdraw from the beta if you trade in. Just do the normal factory reset should be good I’ve never withdrawn from one. If you want you can withdraw from the beta too be safe but I don’t think it would be a issue
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The beta will most probably be over at that point, but if not I've never withdraw from the beta, I just did a regular factory reset and that's all good since the beta is after all a official OS from Samsung and it doesn't void your warranty or anything like that, unlike when you root your phone and install a custom ROM, here it's official
We recommend you to withdraw from Beta / Factory Data reset (Remove any accounts logged in/ remove lock screen...) before you trade-in. However, the main action item is to Factory Data Reset than Beta if you are trading in (for your own security).
