There are many reasons people buy anything and selling any digital item (NFT) is no different.
Some NFTs like Cryptopunks are collectables sought after for their rarity and cultural fit into the crypto space. Collectable NFTs usually have a steady volume of trading happening between buyers and sellers which makes them great for trading.
Other NFTs like Cryptovoxels are more akin to 3D website builders that can be bought and sold with a specific angle for utility. Virtual Land NFTs fulfil a utilitarian and collectable niche.
Unique Creator's NFTs like the ones you can create on Mintable will be as sought after as you can make them based on a number of factors.
Amount of work put into them
Quality of the work put into them.
Utility in relation to goods and services
Added brand value
If you have a low-resolution gif from a random website, most people won't buy it. There isn't any added value in legitimacy that is coming from you turning a Gif it into an NFT.
Successful NFTs sold all have one thing in common:
A high resolution, custom-made, beautiful artwork is much more likely to sell than a random photo of your dinner.
Creating NFTs that are rare means that only a certain amount of them will ever be made. Since NFTs can be traded, rare NFTs can be collected and held for profit later on as a larger audience comes along and has their own take on it's subjective value. Rarity alone is not enough to guarantee that an NFT will retain it's value, and for some specific file types it makes very little sense to limit your own sales.
Giving the buyer something special to access once they buy the item is vital to being a successful seller.
If you are selling artwork and the image everyone can see and copy and paste is the same image that the buyer gets to download, there isn't much incentive for them to download/buy that item.
But if you have a watermarked, lower-resolution version as the public image, and only the buyer can download the 4k high-resolution non-watermarked copy - they are highly incentivized to buy it.
You can give them a .zip file to download with multiple files as well.
Make your item collectible. Give it a reason to be collected and resold or held onto.
If you own a website - use the NFT as a coupon on your store or access a special part of your forums.
Make the item a part of a collectible series, or make it so that collecting 5 of the items grants the buyer something special such as some merch from your brand.
Make sure whatever you make it is beautiful or high quality. Even if your selling a domain name, a beautiful image or video explaining why that domain name is so valuable increases your chances of selling your item by a lot!
Write a good description, include a video or images to explain why your item is valuable. If you just put one or two words for your item, it probably won't sell
This description has a youtube video, a detailed message about the item, and explains what you get access to when you purchase it.