I would not purchase a used 520 unless it were made prior to the latest remodeling... I think the year was either 2017 or 2018. Someone correct me if I'm wrong. The Trek 520's made on or after 2017 have a history of cracks exactly where you have one on yours. This website is filled with stories of these cracks. Someone mentioned that the 520 has a carbon fiber front fork, but that's not true. As part of the redesign in 2017, they replaced the steel front fork with an aluminum front fork.

I have a 2015 Trek 520, and I love my bike. It has the "old" steel front fork, which I crashed and bent, but was able to still ride until I had it replaced with a new steel front fork... thankfully. I'm also a clyde, weighing as much as 220 pounds, standing at 5 foot 6 inches. All that weight with 50 pounds of gear, my 520 held up just fine on a short tour I did a couple of years ago. Thankfully, I'm no longer 220 pounds, and I continue to come down from that.

My 2 cents; good luck with what you do.