Jeff chuckled nervously to himself as he set up the parameters he thought would be good, fun, interesting, or at least exploitable. Not only did Jeff make it so that verbal agreements were enforced by the universe itself, but he made it so the ramifications were only noticed by him and were considered to be the normal state of things for everyone, otherwise, 3rd parties would notice and everyone would immediately stop making agreements all over the world.
Next, Jeff limited eligible trades to what the parties 'have' or 'have control over' even in a loose sense. Enforcing lovers give ownership of the moon away or something like an average 6 year old saying "Let me slap your face and you'll gain 400 lbs of muscle." didn't make sense. This also only applied to agreements between people representing themselves and not entertaining. (Singers, comedians, actors, cosplayers... will not be impacted while performing.)
Jeff made it so that the conversation leading up to an agreement would be forgotten by all except him if it changes reality, Remembering saying you'd happily take on the light sensitivity burden if it meant you'd have your friend's gorgeous blue eyes, when you remember always having blue eyes, would cause issues. Finally, to make it more likely that he'd get what he wants, people will make Jeff counteroffers if they dislike his offer, and go along with him if they like it, but outright refusal will only happen if nothing could ever get someone to part with something.
In the pocket dimension Jeff created, many wishes were granted pretty rapidly. "I'd happily take this!" or "I'd give up this for this!" types of desires in particular. Many trans people physically transitioned overnight after sending pleas out to 'anyone in the universe' and getting an answer. Likewise weight fluctuated between people, teen boys gained and women lost facial and body hair, eye and hair color changed.
Some less than carefully worded offers ("I'd give anything to...") turned children into 'grown ups' which in some cases was 18 and in other cases was 85, and everything in between. Marriage vows were suddenly taken much more seriously. And every so often an elderly man or woman dying of cancer or something, knowing it is their time, wrote a blank check saying, "I'd take on anything to spare others from this level of pain," or something similar as they pass, thereby absorbing countless diseases and ailments, sparing thousands of people in an instant.
All the ways people could randomly agree to mess up major aspects of their life is interesting but this is about a 16 year old named Jeff who is the only person in this universe who knows this rule exists... Jeff may encounter and recognize these situations for what they are, but what happens to Jeff?