We have in Poland monitoring tool for the biggest sales platform called: allegro.pl.
Here is the link: https://tradewatch.pl/index.jsf
Using this tool I can check a lot of things by category. I.e. I'm choosing "watches" and I see:
I use junglescout.com for Amazon
It gives estimates, but I have found it to be at least a good indication of the popularity of the product.
Sometimes they overestimate, but I have found some items they underestimated as well, which is a nice surprise.
Don't know about Ebay
Years ago i used Terapeak for Ebay, but it was acquired by Ebay, i don't know how it is now.