In your example, coin sellers would constantly need to be increasing their total reserves.
If their reserves stay the same (i.e. they are acquiring as many coins as they are selling), there are the same amount of coins in play at all times. There is zero economic benefit to stock more product than you sell. It costs you money. You should have just enough stock so that even at your busiest moment, you don't run out, but preferably you would want to just barely meet that demand. Stocking levels are one of the most important aspects of profitable business.
The "spending their savings" example only works if all coins sellers are depleting their stock, which you just argued against. You disproved your own point.
If the coin sellers don't exist, the same amount of coins would exist overall (since we established coin sellers don't generate new coins), but would be spread among the player base. If anything, this would only cause every player to go up in price or stay the same as now, not down.
Also, coins sellers generate an un-natural amount of Fifa 16 coins being subjected to the 5% EA tax, which actually decreases the coins that are in play with them around as opposed to if they weren't, which again, works against your theory that coins sellers inflate the market.
Coin sellers need a huge stock of liquid coins at any given time to accommodate the new orders coming in. There is no point in investing anything except for what is needed to generate new coins.
I would bet that in a macro sense, roughly the same amount of coins would be in play at any given time, regardless of coin selling.
I think it's quite obvious why that is incorrect.
Coins are generated as people play more and more matches and buy more and more packs. Coins can only leave circulation if people spend them on packs. The reason why we know for sure that the amount of in-going coins is larger than the amount of outgoing Fifa coins is the current inflation.
Another certain piece of evidence is the fact that coin prices are as low as ever.
So, I will take the bet.