My experience with Flipkart exchange has been terrible for all the 2-3 times I did. Though the value was never reduced, but it took me multiple attempts to get the exchange done. (This loop hole is fixed now, but earlier, you could place multiple orders with same device exchange). I used to order multiple orders with Home address, office address, wife office etc with 1 day gap deliveries and get the exchange attempted if it was denied earlier. Got picked up mostly from office address
My first exchange in 2015-16 was seamless. The person who came for exchange got drooled by flawless iphone. Gave me cash (500 more) and I handed over to him.
However, later i started using cashify and similar apps; they do lower the price but it's upon your negotiation skills. If you are confident, you'll get the worth dont budge. I got iphone 8 exchanged few years back which had battery issue. They reduced price by 1500 stating that new battery price. I clearly told, if i am not getting benefit of new battery why will I pay 100% of it. I'll rather get it replaced and sell 2K higher. Eventually only 500 was reduced, as everything else was flawless
Now, I just go to physical shops which deal in used mobiles as they are the eventual takers in many case (especially if the mobile has got second hand market eg iphone, samsungs). Do get expected price; higher than FK, cashify etc. In physical shops, Box and Bill help, as it makes easier for them to sell further as genuine mobile and not stolen. (Sold 2 iphone SEs and got much better price than online exchange)
Have sold twice via OLX aswell to direct individual, got much better price. (Keep realistic price, don't negotiate over chats)
Mostly my sell has been iphone, so its been easy. I generally upgrade/change phone for all family members in 2 years (max 3); so almost every year there's 1 or 2 sells/exchange happening