I was asked ( as a vendor ) to do some of those Fridays, Mondays...Xmas and whatever marketing sales. 
As a person ( not as a vendor ) I feel it a bit not moral selling a tea / teaware for 100$ and then next day , exactly the same tea not being damaged or something what wout justyfy the sale , to sell it for 80$.
It's a spitting it to the face of the customers who have bought the same item for 100$ and they may rightfully feel being robbed 20$ , because I just said " hey , I can also sell it for 80$ ! "  . 
It is a different case if discount is set based on amount ( more buy , cheaper it gets ) or if old green tea for example. But tea  / teaware which has no issue , in my opinion , no reason to discount.
Discount means admitting the failure.  
For the vendors who start to play this game, there is now way back anymore. In fact , it goes down the spiral itnto the more ridicolous discounts like 75% off ,  or  "shipping is even more free than before " ...yeah, you read that right 

  I've seen that one on Reddit once. 
So naturally , you  ( as a buyer ) will wait for the special offers , having like X shops in your bookmarks , subscribed on Newsletters and buy the tea in right timing. That's 100% right , no argue about that. It makes you righfully feel smarter because other "idiots" buying same tea for much higher prices who justyfy it them selves " I don't spend time on research / checking up on sales , and time = money , so I don't feel being cheated ". Yes, also the attitude and can't argue about that. 
I'm paying a subscription for one service every year. I'm 8y customer of that company. During all those years they were rising up the prices every year and I kept paying ( coz need that stuff for our business ) . Last year my friend ( new customer to them )  purchased from them same service for 40% less because Black Friday ...you can imagine how I felt.  
On my renewal ( which was after BF ) I had to negotiate ( by threatening changing other provider ) to get at least 20% discount. Things are bit more complicated with this service , but I was at the point of actually leave and find other one.
This year I took a BF offer to pay the price I paid back 4y ago ( which is the limit I'm willing to pay ..and compare to the other companies ) .