Does anyone have a way to accurately find out which are the top 5 legendary runes that the most number of people are trying to buy in diablo 4 trading on JSP?

Same way you have in mind (I think).
Look up the meta on mobalytics (Mythic Build, Boss Killer Build etc.) and Maxroll Pit Push Build and check which runes they use.

When looking it up, there is a search function on the website. That way you can properly purchase from your RMT vendor accurately and make sure to exploit as best as possible. Hope this helps!

not rmting lol I wanna know which runes are worth converting to other more desired ones cuz i have so many runes that arent selling lol

It's a meme. There's a dude on here that RMT's and exploits the game, then turns around and buys cdkeys with it, and turns that into real money. Unfortunately even though I've reported him getting people banned several times, nothing is done about it.

As far as runes go, I would say Vex/Jah/runes for the 3 Hydra builds out there. However, they are pretty much maintaining value around 20-30fg currently after the dupers blew up the market on day 2.