“First, marketable securities, being income producing assets usually capable of increasing in value, are prima facie purchased and sold by way of investment and not by way of trade. Secondly, a series of purchases and sales may sometimes, if carried out pursuant to a deliberate and organised scheme of profit-making, amount to a trade. Thirdly, it is easier to characterise a series of purchase and sales as a trade in a case where they are made by a trading entity as opposed to an individual. Fourthly, in the case of a trading entity that characterisation is more easily made where the purchasers and sales are substantial in relation to its other activities, all the more so where they are of frequent occurrence and extend over a long period of time …”