How to display a Rules Link only if access to display the linked content was granted?
From the (my) accepted answer to your related question (= How to access node properties of another node using the Rules Link module?), I do remember that apart from the Content Access module, you ALSO use the ACL module.

However, even though via the ACL module you do get some extra Rules Actions (such as content_access_action_user_grant), there doesn't seem to be (or at least I have not found it yet) a Rules Condition that you could use to check that "the current user already bought the node" (as in your question here).

Too bad, if IT IS NOT available, then just MAKE IT available. For that, you can use the Flag. If you then configure a (non-global) flag, you could use that flag like so:


When configuring the flag, only allow it for bundle 'Classified Ad'. And make sure to only grant access to the flag to users with a role like "special role for Classified Ads" (name it what you want), to avoid the link to flag such nodes would be shown to unauthorized users (due to which they could flag the node just using the UI, which you do not want to happen).
Around the time the user buys access to your 'Classified complete' node, perform an additional Rules Action to "flag the 'Classified Ad' node by the current user as bought", and with the machine name of the flag (say) 'node_bought' (pick whatever fits).
Within the Rules Link, use a condition like "Is the node flagged with the 'node_bought' flag by the current user?".


Below is a (rather basic) Rules Component (= condition set (AND) "Rules link: authenticated user Link contact infos condition") with machine name to match your existing Rules Link also, as in your linked question. And which you could use for this (change the machine name of the flag from 'node_bought' to your flag's machine name, prior to importing this):

{ "rules_link_condition_access_to_contact_info_links" : {
    "LABEL" : "Rules link: authenticated user Link contact infos  condition",
    "PLUGIN" : "and",
    "OWNER" : "rules",
    "REQUIRES" : [ "flag" ],
    "USES VARIABLES" : { "node" : { "type" : "node", "label" : "node" } },
    "AND" : [
      { "flag_flagged_node" : {
          "flag" : "node_bought",
          "node" : [ "node" ],
          "flagging_user" : [ "site:current-user" ]
        }
      }
    ]
  }
}


Something I'm unsure about right now (= homework for you to QA-test): as per that "special role for Classified Ads" (see before), the (extra) Rules Action to flag the node may fail (not authorized for current user). If that's the case, then just add these 2 extra Rules Actions as a work around for that:


Grant access for current user to "special role for Classified Ads", right before the Rules Action to flag the node.
Remove access for current user to "special role for Classified Ads", right after the Rules Action to flag the node.