Is it considered gentlemanly to buy things on sale and then mark it up and sell
It depends on how you look at it, it's very difficult to say yes or no to this.
Let's consider the following:
I am looking at Amazon/Flipkart or any other site at random and find a product at deep discount (let's say 3000 vs 6000). I have no intention of using it, but I buy it in anticipation of being able to sell at a profit.
Scenario 1:
You are looking for the same product a month later and find it for 6000. You regret missing the sale. Then you see me selling it for 5k. Because I bought it for cheap, you end up saving 1k, I make a profit of 2k. In this case I'm a saviour.
Scenario 2:
You are looking for the same product and see there's a sale. By the time you reach the sale page, the product is already out of stock. Then you see me selling it for 5k on the forum. In this case I'm a scalper.
Good or bad, basically there's no way you can stop someone from doing this if they want to. With numerous bots and sites which scan for such deals, the only time a genuine buyer can grab a good deal is when there's no shortage of the product. This is what a perfectly competitive market is like, when information asymmetry and access restrictions get removed. The prices are driven by demand and supply. If there's sufficient supply, there's no way for scalpers to benefit a lot. In times of scarcity, no matter what, scalpers will benefit.
And this is not a new thing, it's been happening for long in all marketplaces. When there's scarcity of rice/wheat, hoarders will buy huge quantities, exacerbate the scarcity to drive up prices even further, then sell there stock at huge profits. This happens for onions practically once every 2 years when prices start going to Rs. 100 a kg. When this happens for essential commodities, some times the government steps in, sometimes they don't.
And it is happening right now with oil worldwide. This is how markets work. So stop overthinking it and just let it be.