You are walking a fine line in purchasing an unmarked piece. Generally platinum is marked on the inside with "PLAT" - though this is not an absolute rule. I guess I would ask if you tested this yourself or if you're going on the word of someone else. I would suggest bringing it to a trusted jeweler to have it tested & appraised, if you have not already, before making the purchase.
However, if it does turn out to be platinum, yes, it will certainly hold it's value.