I have a merchant ID can I use Paypal credit?

Where are you talking about buying from?
The option to use PayPal credit is only available once you are committed to making the purchase and you choose it as an option once directed to the PayPal site.
It's just another way of funding your PayPal purchase same as if you chose to use your bank account or credit card to fund it via Paypal.

Anywhere you can pay with PayPal you should be able to select credit from YOUR PayPal account at checkout.

Hope that makes sense.

I haven’t made an offer yet. I’m asking cause a couple of yrs ago ( my last transaction in Agon) I was not given the PayPal credit option and had to switch payment methods. At the time it was asking me for a merchant I’d. There is an item that interest me now And I’m  doing my due diligence.. 

I've had a PayPal Credit account since day one, (actually before, it used to be called "Bill Me Later").  Only time(s) I wasn't able to use it was on overseas sales.  If you have to change currency, you can't use PayPal Credit...AFAIK.