Some auctions will collect from you but it most likely costs but can be useful if you have a lot (no pun intended) to put in the auction. Otherwise, download necessary forms, fill them in and arrange a time to take your stuff to the auction. Be prepared; the seller's premium can add up to about 30% of the hammer price and some auctions may have a minimum premium for each lot. You may get more than you expect as some times the selling price can be quite high, or if only one or two people want it, or worse still only one, then you can get stung. On the other hand if you were buying you would be delighted with your bargain.
The advantage is that you deliver to the auction and that's it; no more hassle selling, posting, fiddling with bank transfer etc. It all depends what you want, quick disposal with a possible good price or a private sale at your price but a possible wait to get it.
Cheers, Phil