A member having a problem with posting a link to a sale on Marktplaats caused me to go have a look for a better way to do it for non-Dutch readers and this appears to be a workaround.
Underneath the image you'll see some links, one of which says E-mail deze advertentie naar een vriend of vriendin (email this advert to a friend) - click that and enter your name & email address in both the from and to parts and click Verstuur (Send). You'll get an email with a link to that page on the site, then simply copy the link into your post on the board.
If anyone knows of a better way please let me know.