How do I enable cookies on iPhone 15 Pro?

Settings > Safari > Advanced >

Block cookies (some web pages might not work)

A cookie is a piece of data that a site puts on your device, so it can remember you when you visit again. To choose whether Safari blocks cookies, tap Settings > Safari > Advanced, then turn on Block All Cookies.

If you block cookies, some web pages might not work. Here are some examples:

• You might not be able to sign in to a site using your correct username and password.
• You might see a message that cookies are required or that your browser's cookies are off.
• Some features on a site might not work.
• 
  

lakija wrote:

amazon or eBay or any app that utilizes the internet,

For your Internet connection.

• Over Cellular: contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you
• Over WiFI: Contact your ISP (Internet Service Provider), the agency who provided the Internet Service on your Modem/Router to you

Connecting to Wifi is first part and then getting connected to Internet is the second

1. If you are connected to a Wifi, doesn't necessarily mean you are connected to internet. If you are not connected to wifi could be a problem of iPhone settings or Router/Modem Settings
2. If you are not connected to internet is the sole issue of Router / Modem Settings. Contact you ISP immediately

eBay is not an Apple product. Contact the app developer here --> App Support 

Amazon is not an Apple product. Contact the app developer here --> App Support

Does it happen only while posting (uploading) or even while browsing (Downloading)?

Probable solutions:

Restart the device. That's it. If issue is not resolved... Force restart iPhone - Apple Support

Try to -> Clear the history and cookies from Safari on your iPhone, iPad ...