It is VERY hard to trade for a legit wish Chansey. I have 3 different ones, but 1 is likely fake and 1 "barely passes" the test by an interpretation of someone who collects events.
Finding a Seismic Toss Wish Chansey (below level 100) that's legit is VERY tough - If you ever find one, snatch it up and please, PLEASE offer me a copy.
In all honesty, Wish Chansey/Blissey is typically not used as Seismic Toss Blissey - seismic toss is nice, but it tends to be a healer (aroma therapy, wish, softboiled, toxic and/or protect someplace in there - often instead of softboiled, which I think is a mistake).
If you're asking whether or not you should "wait and see" if you can find a better chansey... objectively, I'd say it's probably better to err on the side of caution and pick one up. Finding wish chanseys w/ good Ivs is super hard (I think there's ONE on this forum w/ "above average" IVs), but if they're legit then they're usually super-super hard to trade for.