Question- i have suppliers who are also clients. (i buy and sell them) -can i define card of customer or supplier to be both ? client or supplier ?

I'm happy to see you again here in the Community, @userpet.

I have some insights to share with you on how to handle transactions if you have a client, which is at the same time your supplier. 

When a client is also your supplier, this process is called a Barter in QuickBooks Online. A barter transaction happens when you and your supplier exchange goods and services. To record the exchange, you’d enter an invoice and a bill to record the products or services you’re exchanging. 

However, before entering your barter transaction, make sure you've added your barter partner as a supplier (for the bill) and customer (for the invoice). Since you can’t have the same name in both lists, make one of the names slightly different to avoid duplicates.

Let me provide the steps on how to set this up:

Setting up as a Customer

1. Click Sales in the left navigation menu.
2. Go to the Customers tab.
3. Hit on New customer.
4. Enter the necessary customer information.
5. Press on Save.

Setting up as a Vendor

1. Click Sales in the left navigation menu.
2. Go to the Vendors tab.
3. Hit on New vendor.
4. Enter the necessary vendor information.
5. Press on Save.

After that, you'll need to create a barter account to to handle the transactions. 

To do that:

1. On the left panel, tick on Accounting. 
2. Under the Chart of Accounts tab, click on New. 
3. Select Cash and Equivalents under the Category Type drop-down.
4. Choose Cash on Hand under the Detail Type drop-down.
5. Enter Barter Bank Account on the Name field.
6. Click on Save. 

I've got here a great resource that you can scan through to learn more about this procedure:  Learn how to record a barter transaction in QuickBooks Online.

Feel free to reach back out to me here if you have any other questions. I'll be right here for you. have a lovely day!