Well, considering that many performers wear the same clothes on multiple shoots, locations & for different companies, I can only assume these are their own clothes, so I'd guess they simply wear them again (both on & offscreen).
Concerning the dildos...
I'm not 100% sure, but a dildo, vibrator and/or other toy can in be cleaned & used again AFAIK.
If you want to buy Ava Devine's used undies, I suggest you find a way to contact her, go to a site that claims to sell them (it's not important if they're telling the truth or not, since you probably wouldn't know the difference anyway) or wait until she announces she's selling some of her underwear, like Lilith Eve is apparently doing.
I have a spot in my closet were I keep all my used toys, props, etc. that I plan on auctioning. I still need to create the store on my website though I know there are websites (though maybe only one) that will auction them off for you. I have a ton of costumes so I should probably either donate them or auction them (though I really have no idea what someone would do with a costume that I've worn before).