Youāre being quoted simultaneously the best available Buying price and Selling price (respectively the Ask, the lowest price at which some investor is willing to sell that security to you, and the Bid, the highest price at which some investor is willing to buy that security from you).
If you execute a Market Buy, you will consume the Ask (the 71.50 in your example); the price of the transaction is 71.50, and that would be the new LTP (historical price). The new Ask would either be the same (if there are more shares available for sale at this price), or it would move higher if all shares available have been consumed.
Initially the Bid would not change whatsoever, and stay at 69.00 (since no transaction consumed that price, the people that were willing to Buy at this price are still willing to do so, theyāve just been outbid by you!). As more and more buyers consume higher and higher Ask prices, eventually the order book would fill with higher and higher Bid.
In a low liquidity market, there just wouldnāt be any interest to point a higher Bid price from investors, resulting in a very high spread; in essence, the Buying price would keep getting higher and higher, but if you tried to Sell, you would still meet a very low Bid price.
Thatās where a market maker will intervene; as the price rises and rises, the market maker will also post new Bid prices going higher and higher, keeping the spread narrow.
Of course, in this example, if you were to instead Sell your share, the exact reverse scenario would happen; the Bid would get consumed (69.00), which is the price you get, and the buyer also gets. With subsequent Sell orders, the market price (LTP and Bid) would move down and down, and hopefully the Ask would follow suite, with or without MM interventions.
In both these scenarios, when a transaction happens (so when you exchange shares with someone else), both parties receive/dispose their share at the same unique price.
Could you sell and buy your share at that same price? If you were both the buyer and seller in the same transaction, yes, and heck, at any price you want! But in every transaction, you are either the buyer OR the seller, not both. And the fact that you decided to buy a share at 71.50 does not mean that anybody else would agree to buy a share at this price; if you try to sell it right away, chances are the best available Bid will still be 69.00, which was already beforehand the public price of the highest bidder.