There are a couple of factors that control whether your prices are imported from WooCommerce with tax already added.  Firstly, we ask your WooCommerce installation whether your prices already include tax.  This is set in WooCommerce > Settings > Tax as shown here:

Next we need to work out whether the individual product itself is taxable.  For example, the following product is set as Taxable:

From these settings, we can then work out that (a) tax is included in the price of the mug, and (b) the mug has a taxable component, in this case Australian GST @ 10%.  We will import the product at $5.99 and record a $0.54 tax amount.

The product is then sold on the marketplace at $5.99 and the resulting order has the mug line item with $5.99 as the sell amount.

But what if I don't have taxable products?

The price we calculate for the product can be affected as follows (taking the above product as an example)

WooCommerce global tax setting "Display prices in the shop"
Product "tax status"
Price of $5.99 product on import
Includes taxTaxable$5.99
Includes taxNone$5.99
Excludes taxTaxable$6.53 *
Excludes taxNone$5.99

* because the WooCommerce site-wide setting states that the price excludes tax but the product is taxable, the tax amount is calculated and added to the the tax amount.  If this results in an incorrect price, then you can either change the WooCommerce site-wide setting or tell us to treat the price we import as already including tax.  This can be done From Settings > Administration: