Isn't there some sort of bond system? I checked when we did same on another site and it's under some kind of protected bond. Check t and c on site but cant believe they dont do similar. I suppose issue you have is I something goes wrong it's not so easy to find alternatives .
We've done it numerous times using Holiday Rentals - I always go for ones with reviews. You can also see how long they have been advertising on there too. If they did a runner with someone's cash - the ads would be withdrawn immediately.
The villa we like has got a review on Trip advisor - I've contacted the reviewer who's replied and has nothing but good things to say...however the little voice in my head is saying "but what if she's in on the scam too". Good god, I need to get a grip .
I've never been on a package holiday and always rent via Owners Direct or Holiday Rentals. There has been the odd problem (pool filter not working/washing machine on the blink) but they've always been fixed asap.
One tip is that if you are un-certain ring the owners rather than e-mailing them, I feel you get a much better feel for things when you talk to people. In general scammers have lots of scams running at the same time and don't have the resources to answer phone questions.
One other tip - on owners direct if you scroll to the bottom of the page with the villa on you should see how long they have been advertising on owners direct.
Whilst a new advert doesn't necessarily indicate a problem, someone that has been on there for 5 years for example is highly unlikely to be a problem. Owners direct remove any suspisions ads or any with lots of complaints.