But you can use them for fast travel.

Haha, would be nice if life worked that way!
'Yes I bought this thing at the time, but now I found a better thing, can I re-sell the first thing back to you?'
I'd sell back half my costumes I bought over the years since now I barely have opportunity to wear most of them.

If you bought a house literally the day before or something, you may ask Support to maybe be nice enough to refund it as a courtesy, but if you've had the house for a whole don't even bother. (Support has been known to refund some items that player regretted, but doing so after some time would go against every sale principle, and could also be problematic to do with houses (since there might be furniture inside them).

That said, old houses can still be useful to store spare furniture for example, or as free wayshrines as Lugaldu said; with the new update, you can instantly teleport to the zone-entrance of your house (rather than inside your house and having to go outside with a double loading screen), so they essentially act as new wayshrines.