Hi Claire,
I can understand your concern but I wouldn't think there'd be any risk in booking one of the apartments advertised on Owners Direct. After all, if anyone had a problem with such a booking and complained to OD, I imagine the advert would be immediately removed and the advertiser banned.
In order to set your mind at rest, why don't you email the owner and explain your doubts. Or if there's a phone number, ring them up. If the apartment is on a complex, you might be able to do a Google search and contact their Reception to check it out.
The Algarve is fantastic and I hope you will have a great holiday!