Really? Spurs have sold for good prices (Carrick, Berbatov), but they are also near the top of the net transfer investment charts. For more info: http://www.transferleague.co.uk/
I think you're missing the point, Spurs only spend what they can afford.
That they have done very well in the last 10-years in maximising their off-field revenue which allows them to afford to 'invest' in the relatively high player turnover that they've had in the past 5 years, going from a mid-table 'cup team' to a Champs League challenger.
So why am I missing the point? The suggestion was Spurs got to where they are by buying low and selling high, whilst gradually trying to improve the squad (like we are trying to do now). This is not the case, as that link suggests: Spurs have a net transfer spend of over 110 million in the last 8 years, which puts them fourth only behind Chelsea, Man City and Liverpool. I don't know if they are spending beyond their means or not: their turnover is certainly much higher than ours (almost double), so they must be doing something right, especially in the commercial revenue area. Anyway, to suggest Spurs got to where they are (regular CL contenders) by developing the squad through not spending more on incoming players than they got in from transfer fees for outgoing players and simply selling high and buying low is a myth.