When am I allowed to start sending my belongings over?

Well! Suddenly, it looks like we have a house...and everything's gone from totally stalled to moving really, really fast. In fact, it looks like he'll be in the house months before I come over (yeah, that's what I get for dragging my feet on the PETS scheme).

Since he'll have a house, when can I start sending my things over? Any time? After I have the fiancee visa? Or do I have to wait until we're married?

It's likely I'll make at least a couple of trips before the big one. It sure would be nice to bring some stuff with me. If I'm coming over on a visitor's visa, will a suitcase full of knick-knacks be a big red flag to Immigration?

I don't want to get the fiancee visa until I've managed to sell my house, just in case the whole process takes more than six months.

I'm pretty sure you can start mailing stuff over anytime at all.  I wouldn't bring too many household type things or knick-knacks in  my luggage just in case they decide to look inside (which most likely *won't* happen).  What I did was just bring clothes over on my visitor's visit. 

you'll probably be fine, because they probably won't question you too much at immigration in the first place (since you don't really fit the profile of "suspicious young american female trying to settle illegally with her boyfriend.")  A suitcase here and there should be fine.

But be careful about bringing two large suitcases full of stuff when you're coming for say, a week's visit- that *is* a red flag, and I've heard of people getting booted partially because they have too much luggage for their length of stay.  But again, they'll probably only look at that if they question you a lot/are suspicious, which I doubt will happen in your case.