At level 29 he'll be the best 5* player available for a lvl 2 manager. So you might get lucky and have people get into a bidding war for him.
At level 30 the starting bid price for him will be higher, which should make up for extra bids you might miss out on.

Either way, if you're thinking about selling him, sell him before the end of the season. As once he turns 19 next season, his value will probably drop substantially (Although of course you might get lucky with a bidding war if you sell him on the first day or two of the season).

Although if he's a fast trainer, and plays a useful position, you might be better off keeping him. As to buy a replacement might cost you a substantial amount of both money and tokens.