My guess is, from a software/admin point of view, it is easy to implement a change where toggled by a number count, ie. 5 to a 100 to any number X, but difficult to make a change with (1.) a number count AND (2.) length of service in SS.
They would probably have to use Moderators in case of rule change where a member was a member for a long period, where individuals who were members for say, six months, (or whatever length of service was acceptable by Admin), would have to "apply" or make a request for allowance to make a classified article for sale, or trade.
It could be done in software with BOTH, but, it would take some time and expertise and if there isn't already a canned "AND" statement in the module, then lilely won't happen right away.