£50 at the moment doesn't mean much, I'd expect it to go up at least some in the closing moments of the auction. Sometimes decent 'scopes can double or treble the highest bid in the last 2 minutes.
A PM3335 in good condition ought to fetch more than £100, maybe even £150 - obviously whether it does on ebay depends very much on which way the wind is blowing, it's a very fickle market place!
Postage, i.e Royal Mail is going to be very expensive but booking a courier won't be too bad. Use a broker site such as Interparcel and you should get change out of £15 to send most places in the UK.
I'd say £100 would be fair as a BIN price, if you are prepared to wait a couple of weeks for it to sell.
Edit: Another 'scope to use for price comparisons is the PM3055 which is close to the analogue only version of this 'scope. One sold on ebay for £99 recently (though inclusive of postage) - it wasn't exactly in "pristine" condition though.