That field around the rim inside the denticles should be empty. What is that stuff? It looks like melted gold.
The rev looks like the remnants of a mount as Dougmeister pointed out. The obv has a number of tiny bumps and blobs that make me think it's a fake. As a general rule, I assume raw $3 gold pieces are fake until proven otherwise.
Looks like it may have been welded into a mount. I've seen the same pattern a few times on cufflinks and tietacks that were welded into "permanent" settings and then polished for that shiny look. However, if it is indeed a welded coin, the process did some significant damage to the reverse of the coin, not to mention the harsh wiping and then polishing of the coin to bring out the "shiny" The diagnostics of the coin look to be on, but even that isn't exactly telltale of a good coin, especially the $3 which is heavily counterfeited. I'd call it probably good, but personally wouldn't touch it as the damage even to a real coin would be too much to command a tiny premium over melt.
Sorry for the delay in responding. Yes, I removed it from a jewelry mount myself in addition to eight other gold coins from a bracelet. It is legit and not a fake.
Those ex jewelry coins won't grade and unless it is super rare it won't be worth much over melt. Maybe if someone needs a hole filler at a discount. I see the one in the OP is quite rare.
With coins, it is usually wise to not only compare apples with apples (as you've done here), but to take it further and compare those of the same "vintage" (so to speak). Even in cases where no design changes had been made over whatever period of time, different dates can possess different characteristics that can be very important when authenticating.
I just bought one of these in a type set about 6 weeks ago. It had been mounted also. The low mintage doesn't mean a whole lot because so few people collect them by date. They are almost just considered a type coin. Definitely polished and cleaned and highly suspect as a counterfeit but what $3 gold piece isn't? I can see where the mount marks are on the reverse and would suspect that is glue on the reverse but can't really tell from the picture.
So did you guarantee authenticity and no-question returns for 30 days? If not, I will report it to eBay. EDIT: I just checked. No guarantee of authenticity, nor any returns. http://www.ebay.com/itm/1889-3-Gold...558421?hash=item33d5ef3f55:g:bpUAAOSwbURZqyUD
Based on what evidence? Fakes made of good gold were often used in jewelry. So far, your story proves nothing about authenticity.
In addition, I do accept returns on Ebay. It says "no returns" on all of my listings to prevent those that buy something and then decide they dont want it 30 days later. I have honored every return request. In the future 971, please ask before sending threats my way. I have been here for a while and feel that I deserve that.