What do you mean I look up things like Pokemon cards and they go for under $3 but shipping is free. How can they make any profit if they want to send with tracked postage 

A lot of sellers will use them as 'loss leaders'

Loss leaders are products sold below their cost or profit margin to attract customers and encourage them to buy other, more profitable items. The strategy relies on the assumption that the initial loss on the loss leader will be offset by increased sales of complementary or higher-margin products. 

People who buy such items rarely buy just one.

Sellers will make a profit when they purchase more.

If I put postage on an item will people still buy my item. 

If I put postage on an item will people still buy my item. 

That depends on the people. Not if they can get the same item cheaper from other sellers.

Not all items are sent tracked.

Small items like cards can go in a standard untracked envelope @ $1.50

As asked, what is the item number?

Is the seller registered in China?

Are they brand new sellers who may wrongly think eBay pay for postage?

Or the reasons already mentioned 

For things like cards, maybe they are not sending with tracked postage, maybe they are just in an envelope, as dave said.

All the same, I get your point, there wouldn't be a huge amount of profit in it for them if the item itself, with postage, was only $3.

You'd be wanting to make a lot of bulk sales to make it worth your while and maybe with things like cards, they do get a lot of people buying 2 or 3 at a time.