Hello Incredulum
I think you are getting there in working out of betfair works.
You original post is only half correct when you say
my understanding of Betfair is that you are actually actively leaving unmatched bets to be taken by others
This is correct in that an individual leaves an unmatched bet at the odds they like, this bet does not materialise unless there is another betfair willing to take the bet offered at the price offered.
If there is no taker the bet never takes place - ie it is cancelled.
The person leaving the unmatched bet to be matched at specific odds is a bit like the bookie.
The person taking the bet made available is like your average punter taking a bet.
However the person offering the odds is sometimes just your average punter who simply thinks the current odds on offer aren't good enough and would be happy to have a bet if he got a little bit better odds- this is probably the case if you leave sporadic unmatched bets with no formal system in place where you sometimes get the odd bet and sometimes don't
Some individuals however will offer unmatched bets on all/most of the different outcomes for a particular
event and they will be highly likely to make a profit provides that particular event continues to have odds at less than 100% payout for the "punters taking the bet" - this type of activity is similar to that of a bookie that it may amount to a trade under some circumstances.
Obviously there are lots of grey areas in between so none of the above should be taken to mean that one particular type of betting pattern will actually amount to a trade as far as the taxman is concerned.
It is a fact that there are plenty (but low in % terms) of non mug punters out there and the bookies very quickly reduce the size of the bets available to these individuals and betfair make extra charges to some individuals that use their betfair account to make consistent profits from very frequent trading.