Yes, you can use purchased in this sentence. The difference is that "to obtain" something doesn't mean you have to "buy" it. For example: I obtained a new chair from the side of the road. A person was throwing it away. I purchased a new chair from the story. I bought it for a good price.

Both would work, but it depends on what you are trying to say. I can obtain something without purchasing or buying something. So, if you selling something you should use purchase.

Well ,since they are probably selling stuff in this shop ,so they would simply use ' it can be bought " in our shop or even ' you can buy it , or ' we have it ' ,,,, that is the way they would be speaking then. Linguistically correct are both ' purchased' and 'obtained' "Purchased" means you can close the deal and buy it , referring in particular to act of purchase . "Obtained" means it is available here and you can have it .It refers to the availability of the item or good .