Problem with black Friday is plat items take a massive hit in price. If you buy plat expect items to sell more slowly and for 50-80% of normal price. On my world this has lasted for upto and beyond a week as the world overflows with every last person having the same wait and buy idea.
Simple supply and demand.
So if you wait for about 2-3 weeks after double plat to sell you will make far more money. Personally I'm saving up 5-7m to spend on rich people with short attention spans. It's a great way to stock up and profit.
Listen to this ^^
If you get plat, the best way to get money is to wait until the next event to sell chests. There will be two things working against you if you sell chests on black friday
1) it will still be samhain, and by this point pretty much everyone has the fashion they want, suppressing chest prices
2) there are going to be a lot of sellers because everyone is buying plat
I would suggest waiting until at least Yule to use your plat to sell chests, because by that point a lot of the black friday plat will be out of the market, and you can sell Yule chests on the very first day when they are at their highest prices.