Some items you obviously want to buy new -- toilet paper, toothbrushes, socks, and food come to mind. (Yes, those are meant to make you chuckle.) But some items are so quickly dirtied by their natural uses that you might as well go ahead and buy them second-hand, because if they're used, you won't notice the difference by the time you're done with them.
Obviously, this doesn't apply to broken materials -- unless you know you can fix it up, don't waste your money on items that don't work. But as far as tools and items that naturally get dirt and grime on them, you don't need to waste the extra money buying them new.
Garden tools are a perfect example. Check for rust and loose screws, but if the tools seem solid enough, it doesn't matter if they still have some of the previous owner's lawn stuck to them; your lawn will replace that soon enough. Sure, you can rinse them off, but why go pay $30 at your local department store when the garage sale host is selling the same tool for $3? It will serve the same function.
Beach toys will also get ridiculously grubby, especially once you hand them over to your kids, and since the little ones will probably get as dirty as the items, there's no need to be concerned with the fact that they'll be muddied even before they start playing (just from carrying around that old bucket and shovel). Again, watch for cracks and chips, which can cut your child's hands, but for the most part the few dollars or cents you pay at a garage sale are worth the pre-dirtied state.
Work boots are normally worn for purposes in which you are avoiding getting your shoes dirty -- meaning the boots will take the brunt of the mess -- so if they're already a little messy on the bottom, it really doesn't matter. Rubber boots and hiking boots are great garage sale purchases. Just make sure they fit before you shell out the cash; trying to wear too-small or too-big boots can be a disaster.
Tarps serve the purpose of protecting one surface from messing up another. Whether that's to protect your tent from the lawn, to keep paint from dripping onto your carpet, or to save your firewood from the rain, tarps are inevitably going to get dirty. Depending on where you buy your tarps, you can pay as much $20 or more, so if you can get a $2 tarp at a garage sale, it's a great deal.
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