Does modding a blaster count as crafting or repairing in terms of adding hp's?

Are you asking about Artisan's Intuitive Improvements talent? In which case, it simply calls out crafting or repairing, but does not mention modifying. By comparison, Inventor does mention modifying an attachment.

Shoot yes improvements... Ill fix title

Also is there a guide to what a stock battery +2hp would sell for?

Usually you would talk to the DM. Typically, adding hardpoints wouldn't make a weapon more vauble, I would expect you to invest in attachments and the like to fill those points if you were expecting to turn something from a stock to artisan item.

I would start bidding at 50%/75% the items price and have every success add 5/10%, a prime negotiator could really sell this item on for huge profit.

Step 1: Wack it with a hammer.
Step 2: Use intuitive improvements
Step 3: ?
Step 4: Profit?