1) There are lots of Dylan vinyls there , some of them very old . First thing that comes to my head is a 1968 US reissue of John Wesley Harding sold for 28 Euros. Many first editions too. Didn't have enough time to check everything out but i will soon .
Aren't those rare and extremely valuable ? Probably depends on their condition , though they looked fine .
It does depend on condition but there are other variables. It's hard to make blanket statements like, if you see this, then the record is valuable. If you're serious about collecting valuable records, you should buy a price guide.
2) How can you tell if a record is in good condition or not?
Look for scratches. Also make sure the record isn't warped---in other words, is it perfectly flat? I also look for dust & dirt. By itself, this might not be a concern---maybe if the record is cleaned, it might reveal no scratches---but for me, a dirty record indicates that the former owner didn't take care of it.
Is there a way to check their authenticity ? I mean , if they're really that old and not some reissue .
The best way to determine whether you have an old record is to examine the label & familiarize yourself with how the actual record labels have evolved. This web page illustrates how Columbia labels, for example, have changed over the years:http://heroinc.0catch.com/columbia/
4) New vinyl for 25 euros vs used in good condition for 10 euros or so . Should i stick to the new ones if they are available and not take any chances?
New vinyl does seem to be better made.
Once again i apologise for my poor English
Don't. I know very few 18-year-olds whose first language is English who would use words like "authenticity." You're doing fine.