I wouldn't take this and have never taken such a ludicrous risk as buying someone else's position on a game that I wanted to bet on anyway. You do realize that if you've been there when the bets are being tallied you could've saved so much money and still earn arguably even more money in the process without paying so much money as what you did right now. Good on you for not losing it all but 9 times out of 10 this wouldn't be the case, trust me, and you just got so lucky today.
I'd never go so far as to become stupid for gambling to the point that I'm buying someone else's bets in hopes of winning whatever they could. It's like an investor paying/funding a business that doesn't even exist yet in hopes of earning money in the process. It's not gonna happen. Call me condescending or cynical but it's just not for me, and I think a good amount of people in this forum would agree with me as well.
I would not even call it being cynical, this is simply being a realist, whenever someone makes you an offer you always need to think what that other person is winning by doing so, since it is very rare that someone else is going to actually offer you a deal that is beneficial for you and instead they are looking out for themselves at all times, so if I was given this kind of offer I will immediately reject it since my first impression is that I will be disfavored if I did accept it.
The point is that you really need to be rational when you get some offers from other people such as those who offer to be gamblers that you fund, or other things promoting gambling to you without any cooperation, it all depends on you and maybe we can only suggest something like what you suggest that they really need to be rational and ask for some proof of the winnings that the person who promotes, if they cannot provide or show something as proof that they really make money from gambling then obviously you should feel doubtful and suspicious of them.
I think it's quite simple to make a decision, and with you bringing a rational attitude then you will really know at least some facts from the suspicion that is in your mind when they promote something without any evidence that can really be trusted and you will be able to get a conclusion and decision on the considerations that you do by being rational.