As the eligibility requirement is rolled over from month to month, If you were to drop from 1000 subs to 990, you would lose your monetization until you exceeded 1000 again; same with watch time hours.
4000 in past 365 this month, and monetization remains. Less than 4000 watch time hours and the monetization will be lost until watch time hours climb to or above 4000 in past 365 days.
Every Top Contributor on the YT Help Forum is saying that buying or selling channels is against the Terms; so I wouldn't risk it if I were the OP.