Seeing as you asked the question, I'll give you my theory.
If you scan through the posts on this forum, you'll see that the ones that have loads of replies tend to be those asking for opinions and advice - as indeed this one has gone onto 2 pages.
Many of those offering things for sale get no replies at all. So from that, I would surmise that unless you are virtually giving something away, or what you have is rare, in great demand, or has some unique selling point then the chances of you selling it on here are small.
So, if what you are selling is a fairly standard one of it's kind, no matter how good, and you are selling it for the going rate, then no one is going to travel more than a few miles to come and look at it. You say you got her as a house cow, but don't actually say why you aren't using her as such . You don't need electricity to milk one cow, especially when she's got a bull calf to take a good proportion of the milk so once a day milking would suffice. So - is she quiet and easy to milk?
To sell something successfully you would have a better chance if you advertised in something that specialises in selling, whereas this forum specialises more in advice and support. Try Preloved or Farming Ads.
Also - you may be the nicest person in the world to deal with - but phrasing your ad as "£650. No offers" would suggest otherwise and is a bit off putting. It's like when people put "no tyre kickers". I personally never go and look at anything advertised as such.