Doing the maths it's 1185/50 = £23.70 per oz, looking around on the forum that's a decent price in per oz terms considering if buying new would have 20% Vat on top of the dealer premium.
In terms of the size I tend to buy a couple of 25 oz tubes at a time. Some stackers are buying in bigger orders, some might be buying a coin at a time the ease of sale will be dictated by the price you were to offer it at I think. If you have say 100 oz in total including that bar you'd potentially be selling half your stack if you needed cash. If you've a 1000 oz stack then it would be a relatively smal