I will start off by seeking your 2 cents on below matter. I will be very grateful for your help and thanks in advance. 
I requested for my gym membership to be canceled but they continued to charge me. I blocked my card and disputed transaction with my bank who ruled in my favor and credited the amount back to my account. My gym was still trying to charge me every month, I went down in person to the gym, spoke to the manager and showed the proof of cancellation who then sent email to the billing department to stop charging. I thought it was all over, only to find a collection on my credit report. Luckily it's not reported on all three, but just one.
Long story short, I got in touch with the collection agency and sent the proof of cancellation(emails from the gym). They were supposed to check with my gym but they never got back and I never bothered to check again.
Now, I disputed it to my credit reporting agency who after a month wait, ruled in my favor. Most likely because the collection agency did respond in the time provided. The collection was taken off my account now.
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Questions
1) What is the likelihood that it will reappear? I fear that at some in point that collection agency will try to sell the debt to somebody, who may report it fresh.
2) Now that collection is out of my credit report. If I call the collection agency and pay off, just to ensure that the collection agency doesn't sell to somebody and that it won't reappear again in the near future. I am just afraid what if after paying, they report it as paid and reappears!.
3) Who is my actual creditor now? My Gym / Collection Agency? (If I decide to pay off)
4) Earlier I tried negotiating with the Collection agency to remove the debt and I would pay them regardless of whether I owe them any. They were not agreeing to it, perhaps I am a bad negotiator. Is it worth hiring a professional service to do this for me? What would the cost be and any recommendations?
P.S: Even though I feel morally / legally I don't owe these guys any money, the debt is only a couple of hundred dollars and I would just pay it off and get over with it than going through this hassle of credit repair which may cost me more than paying off the collection anyway. Let alone any bag of worms this debt can create later on because I may change my address if I relocate to another state and I may not get any physical mail which may go unnoticed. I don't know if they sue or take this legally. I would definitely fight back If I get to know but that is something I want to avoid.