Nothing stopping you at all as far as im aware, I know people who have done exactly that.

Its well worth having the card so definatly worth the effort to get one, some of the price differences are ridiculous, especially things like bulbs, where I get 80-90% discount on most of them.

Although of course all that does is bring them back to what is a fair price charged by local autofactors in the first place, but the difference is that near me those people dont open on a sunday, and thats normally the day I get most free time to work on my car, so its halfords or end up waiting till next weekend for something I need, so to me my trade card is really useful.

As mentioned above though, on sale items be careful as sometimes the trade price is higher than the sale price, so buy those items in a seperate transaction without using the card.
Sale stuff I normally buy on staff card instead, as that still takes 15% off everything even including sale items (my mrs works there and employees partners get a card too)