The gemara says that if a buyer payes 200 toward a 1000 purchase, and then the seller backs out of the deal, the buyer has the upper hand. He has the right to choose whether he prefers his 200 back or whether he would like 200 worth of the sellers best property. The gemara explains that the buyer can demand 200 worth of the best property of the seller, even if that wasn't the property that he was trying to purchase. why? Because presumably the buyer didn't have the cash to purchase the large piece of property from the seller. Therefore, the buyer had to sell his smaller properties at discounted prices in order to get the money together to purchase the large piece of property from the seller. The selling at a loss is considered "damages" which labels the buyer as a nizuk and therefore entitles him to the "idiyus" of the mazik. Tosafos is bothered that if the buyer is really considered a nizuk because he had to sell his properties at a low price, he should be able to claim the amount of the difference (between the appraised price and actual sale price) from the mazik. Yet, we do not allow the buyer to collect any extra money from the mazik, he is only entitled to the amount that he paid to be returned, just that he can collect this amount from the seller's idiyus. Tosafos concludes that it is really a very loose connection to consider the seller a maik. Tosafos implies that m'doraysa he is really not entitled to idiyus, just that the rabbonon "compared him a little to a mazik" and entitled him to collect the amount he paid from the sellers idiyus.
Perhaps we can suggest another approach. Is the din that a nizuk collects idiyus part of his compensation for incurring damage, or is it a gezeiras hakasuv to penalize the mazik that whenever he pays for a damage he must do so from the best of his property? If we assume like the second approach that the mazik is penalized to pay from his idiyus, even when a mazik indirectly causes damage to a nizuk i.e. grama, he should be penalized to pay from his idiyus, just that there is a din that he is completely exempt from paying any damages for grama. Meaning, he is labeled as a mazik who should be paying idiyus, it just happens to be due to a technicality of grama he doesn't have anything to pay. However, in this scenario, although the seller is only a "goreim" of the actual damage incurred by the buyer, and on that amount he is exempt from paying at all, the seller still is labeled as a "mazik" therefore on the 200 that he is required to return he has to pay from his idiyus.