If you have the SDK, the API documents are included in the AP documentation. You have different interfaces available though, including direct API calls or use of staging table with APIs, From the front-end you have the AP Import utilities and Data Import. With no other details, the best solution will depend on your level of expertise and the application requirements. 

Thanks a lot, where can we get the SDK from? 

We only want to send the accounts payable invoices to sage 500, checking the vendor list and account tax information can also be a part of it. 

Talk to your reseller about getting SDK but that might be overkill given your limited scope. Unless you have in-house development experience with SQL and Sage 500, you should look at alternatives like DI. The APIs are used by DI to process documents populated into the staging tables and includes open AP vouchers as I recall. The code is SQL-based and available to any calling interface but you have to populate the staging tables manually if you don't use an interface like DI or it's direct call UI counterpart, Assisted Company Setup. You could also discuss your requirements with a developer. You also might want to review the DI overview document. The overview documents are available in the UI in help, but I just always go to the normal client installation location (Program Files (x86)\Sage Software\Sage MAS 500 Client) and pull them from the Help folder.

There is no reseller as yet, we have a development team and building integration with sage 500 similar to Sage cloud accounting.