Junk debt buyers buy and sell old debt constantly. However, the seven year reporting period cannot be "restarted" when a new junk debt buyer buys your old debt...unless you make a payment to them.
When a junk debt buyer reports the debt to the credit reporting agencies, they may attempt to "reage" your debt by changing the last date of activity. This is illegal, and you need to immediately report this to the credit reporting agencies if it happens.
You'd think the credit reporting agencies would keep tabs on this illegal practice, since your reports and FICO scores are so important, but that's not what happens. The credit reporting agencies are in business to make money, not help average joe consumers have accurate reports, despite what they may tell you.