Here is an idea:

1. Decide on a rate relative to MtGox (e.g. MtGoxLast [+/-] x%) and declare that the rate at the moment someone "accepts" the offer will be the rate paid.
2. Set up a bitcoin address dedicated to this offer, and require that your buyer send a 10% deposit to the address, to lock in the timing and the rate.  Accept only the first acceptable payment, refund all the rest if there's more than one.
3. If you are afraid that you might get burned at an unfavorable rate because someone buys right at the tip of a downswing, then simply state that you reserve the right to cancel without prejudice any acceptance that was made while the MtGox high was > 105% of the low, over the 24 hours prior to the acceptance.
4. PGP sign the whole thing and publish it.
5. Wait for incoming bitcoins, and then make sure the person who says they sent them can prove the sending address was theirs.
6. When the buyer sends his fiat money, send his bitcoins and return his deposit.

This procedure prevents all the pussyfooting and locks in a specific moment and rate to be used.  I have used it with astounding success for the limited familiarity our community has with it.  If more people did this, we'd have decentralized exchange.

Doing it this way, you aren't stuck with possibly having to accept the bottom end of whatever variation there is in the rate during the time it takes you to negotiate and set up a transaction, versus seeming to offer an uncompetitive rate.  It's effectively a variable-price-pegged-to-Gox Buy-It-Now offer... fixed in terms but variable based on when accepted.

If you will set up an offer like this, I will strongly consider accepting it.  The benefit for me is that at whatever point I pull the trigger, I should have a good idea of exactly what deal I am walking into, the moment I act on it.  It's confidence in the marketplace.

If your in UK you can send WU GBP anywhere that accepts. ie Pakistan

I need around £170 ($277) Western Union which will be paid for with bitcoins. I'm happy to deviate up to 4% of market value.

This will be a recurring thing so regular traders welcome. Even if you can only provide this one time that is still fine Please PM me or post here.