Picking your choice tomato from the store and drying the seed is no problem at all. As a rule I always save seed and use it the following season. The one big issue with getting it from a store, and everyone who grows there own knows it .. store bought tomatos are at best, tasteless. So how can you identify what you do and do not like. If you can track down a store that sell the tom I previously mentioned .. Maskotka .. you will possibly notice the price of at least £3.00 for a small tray, containing just a few toms. They are usualy about £8-9 a kg.
This year I have disposed of all my seeds and am starting afresh, with known certified seed. The problem is if you start with unknown seed for the sake of saving £2.99 you may well have poor results or failures and as the growing window is so short, better to take less risk and be sure of good results with the known seed.
That planting time is only a few weeks away so I wouldn't want to miss that window by finding that my seeds haven't germinated after 3 weeks.
If your tomato's fail and it is no big issue, you can shrug your shoulders and say hey ho, but I will be hoping to have a good few hundred pounds of top quality toms to see me through for 12 months.