Best place to buy used engine?

[Peterf1530D]

I have a 2010 Aveo that I adore and have no intention of discarding. However, it has accumulated an embarrassing number of miles, and when it eventually fails, I plan to replace it with an engine that has fewer miles. I am not willing to spend more than $500.
I have seen various engines listed on Blue Prism Automotive, but I am uncertain whether to trust the platform. Additionally, I have been contemplating the possibility of purchasing an engine from a junkyard.