You aren't supposed to buy first or second class relics. It is said to be a form of simony.

https://www.ebay.com/itm/134243453349?mkevt=1&mkpid=2&emsid=e90001.m43.l3160&plmtId=700008&mesgId=3024&mkcid=8&ch=osgood&bu=44176704746&trkId=274062c8-c4ad-4ed2-a291-9881618ec7a8&cnvId=700003&recoId=134243453349&recoPos=1

I found this and although from the looks of it it looks real with the red seal, I thought it's obvious that there are no relics of the BVM.
The genuine look of the red seal suggests that a church authority approved the relic.
Would the church authority really have thought that this was a genuine relic?

I was looking forward to buying genuine-looking first class relic on eBay, but I hesitate because it could be fake.
I guess I could always just avoid obviously fake ones like the one belonging to BVM, but if they can fake relics of BVM like real ones, how can I believe in the genuineness of relics of the saints?

No chance that it's genuine. I would be surprised if even 1% of the relics listed on the internet are real.

Having the care of first class relics is a serious responsibility, not something to be taken on lightly.

...how can I believe in the genuineness of relics of the saints?

You can be certain that they are genuine if they have a solid provenance, otherwise they are questionable.


Relics are to be used as an aid to devotion not as a talisman, so I wouldn't spend much time worrying about it. If God decides to send one your way, fine, if not, thank Him for not burdening you with the responsibility.

It is forbidden to SELL relics, but it is permitted to "BUY" them (i.e. acquire them for a price) in situations like this ... to get them out of the hands of evildoers.  Who knows who might end up buying these relics and for what purpose?  Your intention in that scenario is not to engage in commerce over relics, but essentially to RANSOM them from evildoers.

So, for instance, while I cannot traffic in human beings, if I were in some country and saw someone selling children (to be used as sex slaves), I would be perfectly justified in buying these children to prevent that from happening to them.  I would myself not be engaged in "trafficking" ... nor would a devout Catholic who was attempting to acquire the relics so they can be returned to their rightful place of honor among faithful Catholics.

I think this would be a good time to show some of the indicators that give away a fake.

Here's an image from the OP listing


It's supposed to be from~1750, but it's spotlessly clean, including the wax seal and the string!

Also the fine print

EBay policy prohibits the sale of human remains and requires a disclosure of what the relics are: these relics are a piece of wood, which are allowed by eBay  

 
 

                                                                        
 
 
The Vicarate of Rome only distributes canonized Saints. The postulators of the various orders, were charged with the cause of the canonization of a saint. Depending on the time, the postulator of the order of the Saint may  have distributed authentic relics: thus the relics will bear the seal of the particular order. The Seal of the Vicarate of Rome is not consistent from century to century, though it is similar to the above. Sometimes it has an even more square shape. While a light strike may be an attempt to cover up a poor stamp, a nice strike of the Vicarate of Rome seal is NOT indicative of an authentic relic, because counterfeiters have been much more careful to counterfeit this seal. Blackened soot marks are often present in genuine relic seals. Cracks in the sealing wax are only indicative of age. Cracks in the seal definitely can occur in larger old Thecas. 19th century Saints, or groups of Saints, with cracking seals, may be a good sign.

Did Joan of Arc leave us any relics?

No saint has ever left us any relics. No saint would ever have the hubris to do it. One essential quality of every saint is humility.

Relics are generally gifts of God to bolster our weak faith. It's like keeping a handkerchief or holy card that belonged to a loved one.