basically, site flipping is you create a site that makes money one way or another and then you set up a multiplier for X amount of months for the profit it makes monthly
say a site of yours do $2,000 a month in revenue and $1500 is profit. You can multiply the profit by 24 or 36 months and you get a $36k or $54k valuation which you can use to sell said site
Closing a site sale is relatively easy compared to make a site earn $2k a month in revenue
First, focus on how to make your site make money then see how you can sell it. THere are plenty of brokers out there that can help when closing a sale