Do the cracks reach to the top tube or into the welds?
If they start at the top of the seat tube and stay above where the seat tube joins the rest of the frame, just drill holes where they end, make sure you have the right diameter seat post, and sufficient seat post length so that its bottom extends well below the junction with the top tube. Aluminum is easy to drill. Any decently sharp bit that's around 1/8" will do the trick. Just make sure you hit the end of the crack with the hole you make.
In effect, bike frames already have a "crack" at the top of the seatpost with a stop crack hole so that the seat post collar can flex the frame and close it around the seat post.
If the cracks go into the weld, or below where the top of the top tube joins the seat tube, or there are more than two of them, you can probably find a better frame for what you're preparing to spend on this one.
A crack at the top of the seatpost isn't particularly dangerous. The racing bike I got when I was 16 eventually broke at the seat lug. It made a sound, and got real flexy. I had the seatpost up too high -- it wasn't inserted far enough into the frame and eventually the leverage cracked the seat tube clean in two, right at the lug below the seat tube. I rode home no problem.
Someone probably rode this bike with minimal seat post insertion, or put a seat post in that was too narrow and just kept going on the bolt, through the shattering sounds, wondering why it wasn't getting tight.