How can I show my offer on a spot?

When you list your item in the Marketplace, and added a location, it will be automatically shown on all locations near the location you added in the add item form.

You can check in the Spots section if it shows on a spot nearby, by going to the spot and then click on the tab Marketplace.

This is a very nice tool if you travel to foreign countries, where kite gear is more expensive then what you paid for. You won't need to take your gear back, can sell if for the same or similar price you bought it for.
The locals will be happy to buy used gear cheaper or buy gear that they cannot buy in their country.
And you will be happy since you don't need to take all the gear back and can buy new gear for more or less the same price.