Sell, Sell, Sell, always sell. As unrealistic as it is, players in FM never have much trouble adapting to the premier league. You can buy any number of LB's that can do a decent job (might not offer Baines' set piece ability) for a fraction of the price. You will have filled a need at AMR and you will have 10 million to spend. I'd recommend accepting the offer, then looking round on the transfer list to see if you can get a cheap option in. If not look at someone like Tremoulinas. If you can't find a replacement you like you can always cancel the deal later on.
Not sure where he starts on this one, but try of Tsepo Masilela, he is not that much worse than Baines and if i remember correctly can be bought for something like 250k. That leaves you 9m + to strengthen the rest of your team, and if you're Everton that means buying a decent striker!
EDIT: Just checked on my San Marino save and Everton sold Baines to UTD at the start for 14.5m and bought Dos Santos for 6.5m. Dos Santos has had 2 and half good seasons (im in Jan 2014) and is still valued at 6.25m. Baines is worth 5.25m and is on the TL for 2.6m. Everton bought Masilela for 180k, and Adrian Aldrete for 3.9m in the summer of 2012 (Aldrete moved to Celtic for 1.8 at the start). Masilela is now worth 3.5m and Aldrete 4.2m. I think this illustrates my point nicely.....SELL