It was in the early 80s. It got to the point where Commodore offered $100 trade-in value towards the C64 for a competitor's machine, and near the end the TI 99/4a was deeply discounted to $49.99. So people would literally go buy a new 99/4a, then trade it in to Commodore to get $100 off. Tramiel likely felt it was worth that loss of revenue to get competitors' machines out of the market and replace them with Commodore's.