If the hulls are A1, no dings, no delam, no leaks,
and if the wings are A1, no welds, have not been rubbed, & good wing tramps,
and if the rudders/rudder castings/connections are A1,
then look at the sails. Likely, being a 1999, they are original and 'blown'.
They may be serviceable for recreational sailing....you make the call...check for delam of the Mylar. Or, factor in the cost of replacement sails.
The last issue is 'does the deal include a trailer?'.
Trailer should have cradles in front and double rollers in the rear....if not, build bunks from 2 x 4 and carpeting. You need to avoid point loading....
Depending on the above, price could be from a low of $3,500 upwards, and $5,700 could well be a fair price.
If the SX was ocean sailed, what is the state of the standing rigging?
In any every, you should replace the shroud anchor pins....just to be safe.
Hope this helps.
1989 Hobie SX18 Sail # 1947
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'