This weekend my wife and I visited two garden centers where, much to our surprise, tomato seedlings about 3 weeks old were on sale (they were struggling to develop their first true leaves). In each case, the seedlings looked starved of light, leggy and generally in poor condition.
Normally, the advantage of buying from a garden center is that much of the hard work has been done. That is, the seedlings have been under lights at a nursery for several weeks, and also at the correct temperature, before being displayed as strong, healthy plants with at least 3 pairs of true leaves. These specimens weren't!
If you want to buy tomato plants at the seedling stage, it would be better to wait until April when the plants for sale are healthier, or sow your own in March or April and choose a variety that suits your particular situation.
More advice on seedlings and growing tomatoes can be found at Nick's website or newsletter.