A stand that knowingly sells partially used park hopper tickets is, by nature, not reputable. The 4-Day park hopper is the shortest length hopper pass available legitimately. If you are only planning on visiting 2 days this visit, keep in mind that a park hopper never expires, so you could purchase a 4-day hopper, use 2 days this visit, and use the remaining two days on another visit.
Buying/selling partially used tickets is against the law (that's why you see "disclaimers" on some booth signs claiming that they only sell/buy tickets for "collectors purposes") It's a scam, and if the tickets are invalid (which is highly likely) you are out the money you spent.
If you only want a 2-day ticket, you will not be able to park hop, and if you want to park hop, you can not buy a ticket with fewer than 4 days on it.