Ive had similar probs with that Interum (whatever they are called)
Might be different to you but I sold a Phone in 2002 for £250 Buy It Now and someone bought, paid, I sent it, job done...3 months later Paypal put my account in negative saying Chargeback (apparently the person who used the card to pay was Unauthorised to use it etc) so I said, I got proof of posting by recorded and I aint giving no one any money till I get my Phone Back.
After months of Bull***t letters saying pay up or else, I stood my ground, "give me my phone back and Ill bring my PP account out of negative..."
They wasnt having any of it. Then it was passed to the first debt agency, I told them the same, they said they would get back to me as they will have to go back to Paypal etc and 2 of the Guys I spoke to even said they can see my point (which they shouldnt reall tell you)
...didnt hear anything for about a year then letters started coming again, I rang, told them the same and once again they said they would need to go back to paypal. 2 years passed, I then get letters form A different debt agency (interum) saying pay up by 7 days or Ill go court.
I told them the same and they said I would have to sort it with paypal and not them--Yeah right, I aint doing your dirty work fella, Get lost, Its down to you to prove me wrong.
Anyway they continued sending letters, rang my house number threatening this that and the other and I told them I no longer want the phone back as I they are for sale at around £30 now so No chance will I be paying a penny and take me to court, Im happy to go wherever as Its out of order that 3 months later I get put in the negative when I did everything and stuck to my side of the transaction!!!
Nothing did happen and I thought that was it. Until November last year they started harrassing me again so I just told them that if they continue to harrass me by letter or phone then Im reporting them and Ill do everything I can to make sure they are dealt with (luckily for me my Sis inlaw is a Solicitor- handy to know
)
My last letter was December saying they are reverting it back to Paypal, I havnt heard since but I know Ill be getting another letter but I aint bothered, I aint paying a penny. Its out of order that Paypal can do what they want and at the time it happened in 2002 Paypal happened to change a few of there rules in their policy.
I wasnt covered under seller protection as I wasnt clued up back then and I was young and seen the £250 and thought Yippee!!lol
So this has been going on 7 years now, They just dont let go!
Your situation is slightly different to mine I see but my point is I stuck to my guns and after the research I did, they just go away......Eventually:D
What exactly was the issue with yours?? You sold ⊗⊗⊗⊗ bags??
Debt collectors 
