Possibly old air bubbles or starting to separate from outer edge from pressure. Usually acrylic is permanently welded from acrylic glue used. Hard to see in photos exactly what it is. Sun may have weakened it outside or anything really.
Do you want 300 plus gallons on your floor for a deal?
My suggestion is fill it up and see after 1 week full then maybe buy it.
Those do look like bubbles, but it looks like its only on the outer edge. You could try welding that edge. Maybe someone with more experience will chime in. I'd be tempted to take a chance for the right price... I mean, if it's like $500 for aquarium + stand... but if it's $2000 I'd walk.