In the US, the First-Sale Doctrine won't apply to software (unlike tangible books and records) so you probably do not have the right to sell your copy of this software to another person.
Since that's not true ownership, I think it can only be described as a license.
But I'll agree that all sorts of shenanigans can, and often do, hide under that generic term. However, "buy" could suggest many substantial rights that are not on offer (most importantly distribution), so it's a bit of a quandary.
The phrase "Buy Now - $9.99, yours forever" might thread the needle. The sale page would still need to include all the legal terms, of course. I think "license" is a necessary word there.