Karma-farming bots exist. It's trivial to make bot accounts and farm karma on meme subs until you're over the threshold, and then go and comment in product-related subreddits.
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.
It does and you can limit posters to in-sub karma. afaik the only way to implement that solution is to write a custom bot though. Reddit's tools for moderation are basically garbage.
The Reddit karma algorithm is already a black box, so there's not much harm in making it more complex, thus:
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.
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.