Ebay will get you top dollar - buyers are more savvy/proficient on Swappa so the prices tend to be closer to what its really worth....i.e. "RARE IN PERFECT CONDITION SAMSUNG S5, RARE MODEL 500USD GET IT NOW" doesn't work on Swappa
But swappa offers a better model in terms of trust between seller and buyer; the seller has to post pictures, get their imei verified, etc. before they can list. The buyer also is more intelligent so you won't get stupid returns, and I'd GUESS (i.e. no proof) you have a lesser rate of buyers filing a complaint and try to rip you off by claiming they never received anything.