I think the moderators could easily prevent this. Just block comments from new accounts or accounts with karma less than some value.
Technical solutions are extremely difficult. A verified purchase mechanism is difficult to do unless you control the purchase platform.
The "easiest" solution is a web of trust, but that has a significant non-technical component by definition.
Measure the general up-votiness of each sub, and scale cross-sub karma based on this value. A +100 post on a sub with 1000 members on a technical topic where people tend to downvote for minor errors is worth way more than a +1000 post on a fast-moving sub that's mostly reposted memes.