What is the buyers loss/gain on a ITM option purchase that doesnt move at all and expires? Does the buyer end up w/a 0 gain and a 0 loss? Or would they somehow take a loss?
If you are able to sell it at the same price as you bought it, then you lose and gain nothing. If you let it expire without selling it then you lose the entire purchase price. If you exercise it since it's ITM, then you gain the profit on the underlying (provided you can close the trade with a profit) minus the purchase price of the option.
ITM means there is going to be an exercise if it's around to expiration. So there would need to have a better sense of details. Is there a dividend during the holding period. In basic theory the buyer would lose the time value, but not the intrinsic. The seller would earn the time value, but the devil is in the details - so throw out some actual numbers and dividend details if any.
"If you let it expire without selling it then you lose the entire purchase price." Wrong. See answer from Jacobson. You have a bad habit of convoluting answers by mentioning unusual circumstances. Ever hear of KISS as in ITM options will be exercised unless there are instructions given not to execute.