I hear this can be done in only a few hours. Is the true ?

Sounds like a scam to me.
They are probably selling some "Get Rich Quick" scheme that turns out to be completely bogus.

They don't.

Those people are likely to be what eBay calls, "power sellers." They sell by volume, have their own stores on eBay and have multiples of listings. I highly doubt they make $10,000.00 in a couple of hours but I have also heard of those who make up to $5,000.00 in one month. But they work at it with listings and quality items. Once in awhile you will see some high end items such as a car or property or some antique. Does that automatically mean that item will fetch thousands? You have to have a buyer willing to pay that much. Most people go to eBay to get the best deal possible.

For example, my favorite fragrance is Dolce Vita by Christian Dior. Not too long ago on eBay there was some up for bid. I placed my bid but knew where I was going to stop. The final bidder paid $60.00 for the fragrance, plus shipping and handling. There are net outlets that do not charge that much and the shipping and handling are free. Serious buyers know their markets.

I would not purchase any type of kit that advertises to sell on eBay. eBay offers resources on their website.

The only people making that type of money arer the large companies that sell to people who list things on eBay.

To make that much you would have to have 1000's of items listed, and if they did not sell, ( and not all items sell) so you would also have eBay fees for every item you list.

I sell some on eBay so unless you are selling a car or a large boat, that sells for over 10,000 you are not.

You can make a few thousand a month if you work at it hard, my sister in law has her own eBay store and makes a fairly good living by selling hand made dresses, but she works 10 hours a day 6 days a week to make that good living also.

I sell a few items a week, but have a few each month that does not sell either.

The trick is to find items that will sell, buy them cheap enough, so remember if you sell something for 100 dollars, and buy it for 70 dollars, then you have to pay a eBay fee, then a pay pal fee to get the money.
You you may only clear about 20 dollars from that sell, and will still suppose to pay federal and state income tax plus self employment tax out of this also.

So even if you made 10,000 after you paid all the fees, about 3500 of that will need to go to pay taxes.