> But later on, now there are only 80 shares out. When buyer 52 comes along to buy a share, there are fewer shares, and fewer sellers. He might have to offer more than Buyer 51 to actually get a share for that reason.

By buying that one share buyer 52 gets ownership of 1.25% of the company. Buyer 51 only got 1.00% of the company when they bought their share. You need to take that into account when comparing share prices.