Here are the problems with this:
1 - on what are you basing "reputable seller" status? How do you know the other pieces were authentic? Tons of people buy (good and bad) fakes everyday and do not even know it. Many counterfeiters will ship a completely different item from what is posted in the listing (which could be a picture of the real item). Is it a well known, highly regarded person from this forum (and you can somehow confirm this)? Now you may be getting somewhere. Don't be fooled by length of years on ebay, or seller feedback. Does the seller have a brick and mortar store with long-standing reputation (ie. Betteridge, etc) or is this an individual seller? How many other VCA items are listed? People sell their inauthentic VCA items in authentic boxes and ribbons all the time (there is quite a market for this, as well).
2 - assuming you do make the purchase, if the seller has not already specifically stated "no returns," you would almost certainly be out of the window for ebay's return policy by the time you send this to VCA (the maison, at cost, not your local store) for verification. This can take months and is still no guarantee. Your local SAs, truthfully, will not even have the skills to authenticate, if they were even allowed. I do not recall the burden of proof for stating something is a fake on ebay these days, but it's probably not worth the risk. To ebay's credit, they have been more supportive on the buyers' ends of things versus the seller; but regardless, the process would likely drag on for weeks and be a hassle.
3 - if you are buying an item <$10K, you are probably not even saving money given the $1500+ price tag of sending the item for verification (and this is the starting price; increases depends on complexity of the piece). Unless you are comfortable with potentially owning an inauthentic item (and not sending to confirm), this is not even smart from a financial perspective. If you ever decide to sell, you are going to run into this problem again of verification (or lack thereof).
4 - the above do not apply to heritage or very old vintage pieces, which you are most likely not seeking to buy on ebay. Stick with reputable auction houses (please know that there are many that are not) and stores (Yafa, etc).
5 - a HUGE pet peeve of mine is that sites like
fashionphile and others continue to show the engravings on their VCA pieces, which does nothing but help counterfeiters.
Some on this site are even guilty of this.... hmmm @calisnoopy.... mods I wish you would delete that thread instead of just closing it. The information is clearly still available and someone recently unearthed that thread).
6 - I'm sure there's other thoughts on this but this is just off the top of my head. Good luck.