In answer to your questions:
1. ClassiPress by default includes the PayPal payment gateway and a manual/bank transfer payment option. You can potentially add your own payment gateway, with some documentation which can be found
here relating to this. You may wish to note though, that in the next major release of
ClassiPress v3.3, there will be changes made to the payments functionality which will allow it the theme to be compatible with other
payment gateway plugins available in the AppThemes marketplace.
2. There is actually no limit to the amount of featured listings that the slider will accept. You need a minimum of 5 for the slider to function correctly, however there is no maximum.
3. The Marketplace products are not all developed by AppThemes, however the developers of products such as ClassiEstate do work closely with our releases to ensure they are compatible. You can always check with them in their own specific section of the support forums before proceeding with any updates if you are not sure. Thanks.