What Apple computer are you using? Are we talking about a 10-15 year old iMac?
Do you have the latest version of whatever browser you are on?
Did you install any new programs or extensions that might use significant memory resources (ram)?
It is possible it is a bug with the eBay website, but it also might be your machine running low on ram for various reasons. Especially with older machines that no longer receive updates to web browsers, they tend to run poorly even if the hardware is otherwise fine.
If you're using a Mac from the current gen (M1, M2, etc) - and you don't have any bloatware or programs taking up major resources, it is likely the eBay website. I am on a more recent Mac, but I haven't noticed any issues.
