It makes perfect sense that the tech lowers both the price to buy and sell ...

Returned items are always bought back at a discount (in earth its 75% meaning you get 25% back on what you sell.)

If I'm a wholesaler I buy at a cheaper rate but that also means that I only get 25% of the amount I paid for the buy back.

its like this
buy = base price*mil tech
sell = (base price*mil tech)*(return rate)

return rate: for all but bu = 25% bu= 8% (actually 8/90)
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Though it may be a little nicer if mil tech shrinks the differential between the 2 prices

buy = base price*mil tech
sell = (base price*mil tech)*(return rate *(2-mil tech))

for clarity return rate + return rate*mil tech
= 1 + (1-mil tech)
= (2-mil tech)

the best mil tech rate is ~ 80% which would increase the buy back to 30% from 25%
(0.25 * (2-0.8) = .25 * 1.2 = .30)