Algorithmic maximum = 20,999,999.9769 BTC [
https://en.bitcoin.it/wiki/Units ]
what will happen to them ?
and can they used for anything else ?
and can the sellers of them do anything to prevent losing money ?
By then the fees would be so much larger than the reward. People who mine would be operating their miners for the fees and not the block rewards. When the last satoshi is mined nobody would even notice that the reward has disappeared and they won't suddenly stop mining.
So nothing abrubt would happen to the value of mining hardware when the time comes.
Also, since this is so far into the future do you really care? Today's mining hardware would have been long sent to the scrapyard by then.
Fees could only get bigger under two conditions.
1. Getting a transaction included into a block is judged by the fee market.
Larger fee= fast confirmation | no fee=no/very slow confirmation etc. messing with the block size could damage the fee market as getting transactions included into blocks would be easier.
2. There's enough demand.
With the current use, the fee market is barely moving. Expecting that the fees will get bigger implies that bitcoin's use will increase and that it will also happen with transactions that are not off-blockchain
