Selling the debt does not in anyway impact either the SoL or the date of last activity. Collection Agencies claim it does as an excuse to re-date debt but they know they can't and that all they are doing is trying to annoy / extort payment of a non-collectible debt.
I'm currently in the process of suing a collection agency for willfully engaging in such practices. It is actually quite appalling how much re-dating happens in Canada and with both Equifax's and TransUnion's implicit consent. I would never recommend that someone pay a debt in such a situation. File a complaint with the Registrar for your particular province and if the collection agency is particularly evil just sue them. What they are doing is a clear violation of the Collection Agencies Act and they know it. They just assume that people with credit issues don't have the resources to do anything about it.
Also as much as I hate to give Equifax any money as they are as much part of the problem as collection agencies I would sign up for Credit Watch if there is any concern of collection agency doing something like this. $13/month is worth having same day notice of anyone doing anything with your credit. It is just unfortunate that the funds will go to Equifax.

