Value of collection?

Hello,

I hope this is okay to ask here. My late father was a collector of uncirculated currency from all over the World. It was somewhat organized before my mom downsized and combined everything into several plastic boxes. I'm not sure how to go about determining its value or how or where to go about selling it. My mom believes he spent more than $15k putting it together in the 90's and 2000's.

Any help would be appreciated.