I bought electronics parts on eBay once upon a time
for servos ( or any item) read the Listing very carefully and check the sellers feedback score. Also, check his feedback to see what other people said about the items he sold.
Email the seller with any questions
Also, try to buy all the parts from one place to save on shipping . ( i buy all my electronic parts from allelectronics.com)
if you are unsure of a listing just post the link to the listing and someone ( including me ) will check it out to see that its good.
,Eric
ALSO REMEMBER ABOUT FREE SAMPLES FROM COMPANIES