It depends if your priority is getting value or getting rid.
The downside of math(s) trades is that you don’t free up shelf space, although the upside is that you tend to get better value in return.
I also found that to use the UK games trade Facebook group, you had to have a pretty thick skin, because it seemed to be full of people with no desire to do any trading but to berate sellers’ offerings instead.
My other difficulty is that I know that I’m simply not organised enough to deal with the admin of postage and packaging. I’ve been toying with the idea of listing things on BGG Marketplace but again the admin puts me off. Which is silly, I know, because I’ve bought dozens of second hand games online.
My preferred method is to take my games to bring-n-buy sales at conventions.
There’s nothing stopping you doing a mixture of all of the above of course.