i can't see the buying and selling of private individuals of a few thousand stock would make a great impact. such individuals, as felix said, would be aiming to take a profitable ride on a wave generated by institutional buyers and the like who buy in much bigger chuncks.
the main impact of t trades as i have experienced it is that of creating an opaque situation. you can see this when a stock has risen and you look at the trades. you may find a few thousands of buys and a similar magnitude of sells and then a couple of millions in the unknown zone!
the lse is obliged to declare trades and the prices at which the said trades have been executed as well as time of execution. however, the lse is not obliged to declare if the trades have been buys or sells.
if you look at the advfn trades and their classifications into buys and sells, such classification is done by the advfn software. the sofware would look at the price of execution and the time of executions and classify the trades into buys or sells. if a trade price does not fall clearly into either category price, and if you are a big buyer you can negotiate your own price, then the trade is thrown into the unknown category.
