Which network and which currency does the marketplace use?

Polygon, and two different tokens that do two different jobs:

• CRETA is the money. Listings, bids and sale prices are all denominated in it.
• POL pays the network fee on every transaction. It goes to the Polygon network, not to Creta.

So you need both, and a wallet with CRETA but no POL can look but not act. Your wallet must also be set to Polygon — if it is on another network, switch before you try to bid.