IGST and place of purchase

I am an ecommerce retailer with my GST registration and place of business in Haryana. I buy goods from across the country. In most cases, I physically go to the state (say Tamil Nadu), purchase goods and get them to Haryana and then put them online for sale.
Now suppose I buy from a dealer in Tamil Nadu. Should he charge SGST + CGST or IGST? If he charges SGST, we will obviously not get ANY input credit for the SGST charged. Can’t we ask him to charge IGST instead of SCGST+CGST?
Also, suppose the dealer directly ships the items to Haryana (even though I have physically purchased the goods from Tamil Nadu), can I then ask the dealer to charge IGST?
Can I, with a GSTN of Haryana, get ANY input benefit for the SGST charged in other states?