yes , diamond has minimums (retail $ amount that must be ordered)
I think the Diamond minimum monthly $$$ amount is $450 (retail costs).
Honestly, if you can't spend less than $450 a month on new comics/trades, don't sell them.
Here's a link where discounts and minimums are mentioned. I have no idea if this information is out-of-date or not. (it looks like the minimum is $425 but may not include retail costs for all items when figuring out that total).
https://retailer.diamondcomics.com/support/retailer_docs/newaccts/diamondtos.pdf