Where to sell bikes?

Without a receipt it looks like a tourist trying to sell a stolen bike, that's most likely why you're getting lukewarm reception, if you can show people the receipt for the bike etc and let people know in the description I'm sure more people would be open to it.

Are you listing it in a group or just on the marketplace? You might have better luck trying some more specific groups like Reiðhjól til sölu notuð og ný or brask og brall.is. However, do bear in mind what u/celezter said. People may be suspicious, so keep that in mind when writing your post.