You don't really need to go to marketplace to get a Developer Access Key. You can get it from the table DEVACCESS using transaction SE16.
To request for a new developer key, follow these steps:
Log on to the URL http://service.sap.com/sscr with your SAP S-user
Click on Register Developer
Enter the user name(s) and select the Installation for which the developer key is required
Click on Register
The key will be displayed in the same screen or you can check from Developer Registered by Me or in DEVACCESS table as described earlier.